gdb: int to bool conversion in find_memory_regions API

Perform int to bool conversion for find_memory_region_ftype function
type.  This function type is used in the find_memory_regions API, both
target_find_memory_regions and target_find_memory_regions.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>