gstmacros.h: Fix restrict definition on MSVC

Turns out it's exposed as `__restrict`, not as `restrict`.

gstreamer/gstreamer!95 (comment 120782)
8 jobs for fix-restrict-take2 in 17 minutes and 40 seconds (queued for 28 seconds)
Status Job ID Name Coverage
  Preparation
passed #128634
gst indent

00:00:16

passed #128633
manifest

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  Build
passed #128635
build fedora x86_64

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  Test
passed #128636
check fedora

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  Full Builds
passed #128637
build android arm64 api28

00:06:32

passed #128639
build cerbero cross win32

00:09:20

passed #128640
build cerbero cross win64

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passed #128638
build cerbero fedora x86_64

00:07:56