gstmacros.h: Fix check for 'restrict' keyword

MSVC also defines it as a keyword. Fixes build errors in projects that
include MSVC's xkeycheck.h which ensures that keywords aren't overriden
with a define.
8 jobs for fix-restrict-checking in 36 minutes and 37 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Preparation
passed #126716
gst indent

00:00:34

passed #126715
manifest

00:00:22

 
  Build
passed #126717
build fedora x86_64

00:04:37

 
  Test
passed #126718
check fedora

00:03:48

 
  Full Builds
passed #126719
build android arm64 api28

00:03:37

passed #126721
build cerbero cross win32

00:09:23

passed #126722
build cerbero cross win64

00:06:40

passed #126720
build cerbero fedora x86_64

00:07:33