build-sys: Bump gettext dependency

Needed for distcheck to work with a checked-in pulseaudio.pot. Not
updating to the very latest (0.21) to not force the dependency bump to
something "too new" on distros.

configure.ac

@@ -101,7 +101,7 @@ PKG_PROG_PKG_CONFIG
 
 # gettext
 
-AM_GNU_GETTEXT_VERSION([0.19.3])
+AM_GNU_GETTEXT_VERSION([0.19.8])
 AM_GNU_GETTEXT([external])
 
 GETTEXT_PACKAGE=pulseaudio