menuselect: invalid test for GTK2

configuire.ac was only checking for the existence of pkg-config
and not the gtk2 package itself.  Now it calls AST_PKG_CONFIG_CHECK
for gtk+-2.0.

ASTERISK-26356 #close

Change-Id: I93e9d0166341f0e7f84b52955bb6f81da42f2ef6