mips-tdep.c: Wrong signness for local holding PC register.

Addresses on MIPS are signed, and we're indeed using
regcache_cooked_read_signed to read the PC, but, we're passing it the
address of an unsigned variable, which triggers a -Wpointer-sign
warning.  I've chosen to change the variable's type.  I believe this
will end up being the same (though I can't test it).

gdb/
2013-04-19  Pedro Alves  <palves@redhat.com>

	* mips-tdep.c (mips_read_pc): Change local 'pc' type to LONGEST.