libmtp: fix built against `gettext-0.25`
Without the change the build fails as:
configure.ac:265: the top level
configure:14220: error: possibly undefined macro: AC_LIB_PREPARE_PREFIX
If this token and others are legitimate, please use m4_pattern_allow.
See the Autoconf documentation.
authored by Sergei Trofimovich and committed by Alyssa Ross be84b997 36030ba7