gmp: fix build on AArch64

Include an upstream patch to fix the build problem on AArch64.

Fixes:

 http://autobuild.buildroot.org/results/1fb1b3678277e6b0cef220405c54ffb8299c9da1/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>