Fix declaration of mips_pc_is_mips

A build where CORE_ADDR is not the same as bfd_vma pointed out that
mips_pc_is_mips is declared using bfd_vma as the parameter type, but
defined using CORE_ADDR.  This patch fixes the declaration.

gdb/ChangeLog
2020-02-19  Tom Tromey  <tromey@adacore.com>

	* mips-tdep.h (mips_pc_is_mips): Parameter type is CORE_ADDR.