Revert "Make oom_score_adjust_is_valid() static"

This reverts commit 6bf3c6c900.

src/basic/process-util.c

@@ -1037,6 +1037,10 @@ bool is_main_thread(void) {
         return cached > 0;
 }
 
+bool oom_score_adjust_is_valid(int oa) {
+        return oa >= OOM_SCORE_ADJ_MIN && oa <= OOM_SCORE_ADJ_MAX;
+}
+
 unsigned long personality_from_string(const char *p) {
         int architecture;