.gitignore: ignore copies of gettext message objects used for testing

If gettext() et al. come from musl libc, ./configure will make a copy of
ext/gettext/tests/locale/en_US.UTF-8 without the ".UTF-8" suffix. We want
to ignore the copy.

.gitignore

@@ -177,6 +177,9 @@ php
 /ext/*/configure.ac
 /ext/*/run-tests.php
 
+# Generated by ./configure if libc might be musl
+/ext/gettext/tests/locale/en_US
+
 # ------------------------------------------------------------------------------
 # Generated by Windows build system
 # ------------------------------------------------------------------------------