selftests/bpf: Fix memory leak in extract_build_id()

getline() allocates string, which has to be freed.

Fixes: 81f77fd0 ("bpf: add selftest for stackmap with BPF_F_STACK_BUILD_ID")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200429012111.277390-7-andriin@fb.com

tools/testing/selftests/bpf/test_progs.c

@@ -351,6 +351,7 @@ int extract_build_id(char *build_id, size_t size)
 		len = size;
 	memcpy(build_id, line, len);
 	build_id[len] = '\0';
+	free(line);
 	return 0;
 err:
 	fclose(fp);