Ada, libgnarl: Fix s-taprop__posix.adb compilation.

Bootstrap on Darwin, and likely any other targets using the posix
implementation of s-taprop was broken by commits between r15-2743
and r15-2747:
s-taprop.adb:297:15: error: "size_t" is not visible
s-taprop.adb:297:15: error: multiple use clauses cause hiding
s-taprop.adb:297:15: error: hidden declaration at s-osinte.ads:58
s-taprop.adb:297:15: error: hidden declaration at i-c.ads:9

This seems to be caused by an omitted change to use Interfaces.C.size_t
instead of just size_t.  Fixed thus.

gcc/ada/ChangeLog:

	* libgnarl/s-taprop__posix.adb (Stack_Guard): Use Interfaces.C.size_t
	for the type of Page_Size.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>