selftests/bpf: Install btf_dump test cases

The btf_dump test cannot access the original source files for comparison
when running the selftests out of tree, causing several failures:

awk: btf_dump_test_case_syntax.c: No such file or directory
...

Add those files to $(TEST_FILES) to have "make install" pick them up.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-6-jean-philippe@linaro.org

tools/testing/selftests/bpf/Makefile

@@ -46,7 +46,8 @@ endif
 
 TEST_GEN_FILES = test_lwt_ip_encap.o \
 	test_tc_edt.o
-TEST_FILES = xsk_prereqs.sh
+TEST_FILES = xsk_prereqs.sh \
+	$(wildcard progs/btf_dump_test_case_*.c)
 
 # Order correspond to 'make run_tests' order
 TEST_PROGS := test_kmod.sh \