gdb: fix 32 bit build

The recent commit dbbb9cfd37 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>

gdb/solib.c

@@ -1230,7 +1230,7 @@ info_linker_namespace_command (const char *pattern, int from_tty)
 	  break;
 	}
       uiout->message
-	(_ ("There are %ld libraries loaded in linker namespace [[%d]]\n"),
+	(_ ("There are %zu libraries loaded in linker namespace [[%d]]\n"),
 	 solibs_to_print.size (), ns);
       uiout->message
 	(_ ("Displaying libraries for linker namespace [[%d]]:\n"), ns);