gdb: add host-texinfo dependency for Git versions

Just like for binutils, gdb versions pulled from Git want to
regenerate their documentation, and none of the MAKEINFO tricks we've
tried worked properly, so we're simply adding host-texinfo as a
dependency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>