Cherry pick cdm udc-widevine-release changes to udc-widevine-dev to be in sync with 18.3 release

Merged from go/wvgerrit/178231

Bug: 290252845
Test: WVTS tests seem to be running and passing
Change-Id: Ifff9123a73e173e835a6e89ba7c2760e1cd500fd
(cherry picked from commit 6889845d2e7e24f22c00b333335c34259b3fc96e)

libwvdrmengine/cdm/util/include/string_format.h

@@ -13,6 +13,11 @@ namespace wvutil {
 #endif
 bool FormatString(std::string* out, const char* fmt, ...);
 
+#ifdef __GNUC__
+[[gnu::format(printf, 2, 0)]]
+#endif
+bool VFormatString(std::string* out, const char* fmt, va_list vlist);
+
 }  // namespace wvutil
 
 #endif  // WVCDM_UTIL_STRING_FORMAT_H_