selftests/bpf: Add RESOLVE_BTFIDS dependency to bpf_testmod.ko

bpf_testmod.ko sometimes fails to build from a clean checkout:

    BTF [M] linux/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.ko
    /bin/sh: 1: linux-build//tools/build/resolve_btfids/resolve_btfids: not found

The reason is that RESOLVE_BTFIDS may not yet be built. Fix by adding a
dependency.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20230403172935.1553022-1-iii@linux.ibm.com

tools/testing/selftests/bpf/Makefile

@@ -201,7 +201,7 @@ $(OUTPUT)/sign-file: ../../../../scripts/sign-file.c
 		  $< -o $@ \
 		  $(shell $(HOSTPKG_CONFIG) --libs libcrypto 2> /dev/null || echo -lcrypto)
 
-$(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])
+$(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(RESOLVE_BTFIDS) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])
 	$(call msg,MOD,,$@)
 	$(Q)$(RM) bpf_testmod/bpf_testmod.ko # force re-compilation
 	$(Q)$(MAKE) $(submake_extras) RESOLVE_BTFIDS=$(RESOLVE_BTFIDS) -C bpf_testmod