qt5imageformats: fix AArch64 compile

qt5imageformats: fix AArch64 compile

Fix libwebp.pri arm64/neon detection using upstream patch [1].

Fixes bug reported by Uwe Hermann [2] and [3], [4], [5]:

  .../build/qt5imageformats-5.6.1-1/src/3rdparty/libwebp/src/dsp/dec.o: In function `VP8DspInit':
  dec.c:(.text+0x2b28): undefined reference to `VP8DspInitNEON'
  .../build/qt5imageformats-5.6.1-1/src/3rdparty/libwebp/src/dsp/enc.o: In function `VP8EncDspInit':
  enc.c:(.text+0x1970): undefined reference to `VP8EncDspInitNEON'
  ../build/qt5imageformats-5.6.1-1/src/3rdparty/libwebp/src/dsp/lossless.o: In function `VP8LDspInit':
  lossless.c:(.text+0x2b18): undefined reference to `VP8LDspInitNEON'
  .../build/qt5imageformats-5.6.1-1/src/3rdparty/libwebp/src/dsp/upsampling.o: In function `WebPInitUpsamplers':
  upsampling.c:(.text+0x41e0): undefined reference to `WebPInitUpsamplersNEON'

[1] https://code.qt.io/cgit/qt/qtimageformats.git/patch/?id=e4c1d9ece74de212ebe5c55b134de7cb23469e2d
[2] https://bugs.busybox.net/show_bug.cgi?id=8986
[3] http://autobuild.buildroot.net/results/0e5/0e577a70bb88e2ba685ccde28576a6857c99876c
[4] http://autobuild.buildroot.net/results/c85/c85f0445151171548cf7c39999ef00d1638871e0
[5] http://autobuild.buildroot.net/results/ae7/ae74f93f09e61151553a54b9d3a6b19946e42938

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>