Prevent secondary warning from diagnostic tweak in gthr-vxworks.h

Prevent secondary warning from diagnostic tweak in gthr-vxworks.h

Within gthr-vxworks.h, we prevent C++ errors from missing
declarations in some system headers by prepending their inclusion
with a

    #pragma GCC diagnostic ignored "-Wstrict-prototypes"

But Wstrict-prototypes is internally registered as valid for
C/ObjC only, not C++, and this trick in turn triggers a Wpragma
warning with -Wsystem-headers.

This change just arranges to ignore the secondary warning locally.

2021-02-03  Olivier Hainque  <hainque@adacore.com>

	* config/gthr-vxworks.h: Prevent Wpragma warning for the
	pragma diagnostics on Wstrict-prototypes.