MagickUtils: Add #pragma to silence IM6 warning

[ferdnyc]

Stupid ImageMagick 6 has a code boner in its header files that produces warning messages at every #include, if -Wextra is enabled during build:

In file included from /usr/include/ImageMagick-6/magick/memory_.h:22,
                 from /usr/include/ImageMagick-6/magick/MagickCore.h:125,
                 from /usr/include/ImageMagick-6/Magick++/Include.h:45,
                 from /usr/include/ImageMagick-6/Magick++.h:10,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/include/Qt/../MagickUtilities.h:39,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/include/Qt/../Frame.h:56,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/include/Qt/../RendererBase.h:34,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/include/Qt/VideoRenderer.h:34,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/include/Qt/VideoRenderWidget.h:37,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/include/Qt/PlayerDemo.h:41,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/build/src/openshot_autogen/RMFXSSRFNF/moc_PlayerDemo.cpp:10,
                 from /home/ferd/rpmbuild/REPOS/libopenshot/worktree-valgrind/build/src/openshot_autogen/mocs_compilation.cpp:2:
/usr/include/ImageMagick-6/magick/memory_.h: In function ‘MagickCore::MagickBooleanType MagickCore::HeapOverflowSanityCheckGetSize(size_t, size_t, size_t*)’:
/usr/include/ImageMagick-6/magick/memory_.h:98:20: warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
   98 |   assert(extent != (size_t *const) NULL);
      |                    ^

...and so on, every time Frame.h is #included. (Which is a lot.) A #pragma disables the not-of-any-use-to-us warning.

[codecov]

Codecov Report

Merging #535 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #535   +/-   ##
========================================
  Coverage    48.79%   48.79%           
========================================
  Files          129      129           
  Lines        10034    10034           
========================================
  Hits          4896     4896           
  Misses        5138     5138           

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[ferdnyc]

Merging as inconsequential, in the grand scheme of things.