util: add missing extern C

This code is included in c++ code via disk_cache in theory,
in practice it never has been.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6035>

src/util/build_id.h

@@ -24,6 +24,10 @@
 #ifndef BUILD_ID_H
 #define BUILD_ID_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #ifdef HAVE_DL_ITERATE_PHDR
 
 #include <stdint.h>
@@ -41,4 +45,7 @@ build_id_data(const struct build_id_note *note);
 
 #endif
 
+#ifdef __cplusplus
+}
+#endif
 #endif /* BUILD_ID_H */