Bug 1714658 - Add missing nsString include to process_util_linux.cc. r=gsvelto

Fixes coverage builds (--enable-coverage) when MOZ_ENABLE_FORKSERVER is not
set. This is the case for Thunderbird.

Differential Revision: https://phabricator.services.mozilla.com/D116885

ipc/chromium/src/base/process_util_linux.cc

@@ -13,6 +13,10 @@
 
 #include "algorithm"
 
+#if defined(MOZ_CODE_COVERAGE)
+#  include "nsString.h"
+#endif
+
 #if defined(MOZ_ENABLE_FORKSERVER)
 #  include <stdlib.h>
 #  include <sys/types.h>