gdb: fix 32 bit build

gdb: fix 32 bit build

The recent commit dbbb9cfd3708a5b09b449c6cbc4d74dfec13904d added a
message using %ld to print an std::vector::size, which is of size_t
type. on 64 bit machines, size_t will be an unsigned long int, making
%ld work just fine, but on 32 bit ones, size_t will be unsigned int,
which causes the build to fail.

This commit fixes that by using %zu instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32901
Tested-By: Luis Machado <luis.machado@arm.com>
Approved-By: Luis Machado <luis.machado@arm.com>