gdb: don't force makeinfo programs for GDB_FROM_GIT variants

Fixes:
http://autobuild.buildroot.net/results/16d/16d4a83eb1e1b9d65641de47f2a2bc5d8ab18bdd/
http://autobuild.buildroot.net/results/df6/df64fe6e834f016267f522f089e00a08b1fd893e/



The GDB_FROM_GIT variants unconditionally use makeinfo, so ensure our
host-texinfo version is used instead of the missing script.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>