selftests/bpf: Fix conflicts with built-in functions in bpf_iter_ksym

Both tolower and toupper are built in c functions, we should not
redefine them as this can result in a build error.

Fixes the following errors:
progs/bpf_iter_ksym.c:10:20: error: conflicting types for built-in function 'tolower'; expected 'int(int)' [-Werror=builtin-declaration-mismatch]
   10 | static inline char tolower(char c)
      |                    ^~~~~~~
progs/bpf_iter_ksym.c:5:1: note: 'tolower' is declared in header '<ctype.h>'
    4 | #include <bpf/bpf_helpers.h>
  +++ |+#include <ctype.h>
    5 |
progs/bpf_iter_ksym.c:17:20: error: conflicting types for built-in function 'toupper'; expected 'int(int)' [-Werror=builtin-declaration-mismatch]
   17 | static inline char toupper(char c)
      |                    ^~~~~~~
progs/bpf_iter_ksym.c:17:20: note: 'toupper' is declared in header '<ctype.h>'

See background on this sort of issue:
https://stackoverflow.com/a/20582607
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12213

(C99, 7.1.3p1) "All identifiers with external linkage in any of the
following subclauses (including the future library directions) are
always reserved for use as identifiers with external linkage."

This is documented behavior in GCC:
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-std-2Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221203010847.2191265-1-james.hilliard1@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/bpf_iter_ksym.c

@@ -7,14 +7,14 @@ char _license[] SEC("license") = "GPL";
 
 unsigned long last_sym_value = 0;
 
-static inline char tolower(char c)
+static inline char to_lower(char c)
 {
 	if (c >= 'A' && c <= 'Z')
 		c += ('a' - 'A');
 	return c;
 }
 
-static inline char toupper(char c)
+static inline char to_upper(char c)
 {
 	if (c >= 'a' && c <= 'z')
 		c -= ('a' - 'A');
@@ -54,7 +54,7 @@ int dump_ksym(struct bpf_iter__ksym *ctx)
 	type = iter->type;
 
 	if (iter->module_name[0]) {
-		type = iter->exported ? toupper(type) : tolower(type);
+		type = iter->exported ? to_upper(type) : to_lower(type);
 		BPF_SEQ_PRINTF(seq, "0x%llx %c %s [ %s ] ",
 			       value, type, iter->name, iter->module_name);
 	} else {