gdb: Reenable makeinfo for GDB from git

This effectively reverts commit 21aaa85 for GDB versions that use Git
instead of upstream tarballs. When building GDB from Git it is required to
compile .info files from .texi files, otherwise build fails.

This fixes:
http://autobuild.buildroot.net/results/df6/df64fe6e834f016267f522f089e00a08b1fd893e/
http://autobuild.buildroot.net/results/95f/95fa2904f81a114abfa337dcce141b7c87d5a52c/

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>