mplayer: add patch to fix altivec usage on gcc 8.3

Mplayer contains ffmpeg 5.x which added some new code, breaking
altivec builds by default. Ffmpeg itself is not broken because it
is compiled with -std=c11, but mplayer does not follow this, so
patch it in the internal copy.

[ci skip]

srcpkgs/mplayer/patches/altivec-gcc83.patch

@@ -0,0 +1,18 @@
+--- ffmpeg/libavutil/ppc/util_altivec.h
++++ ffmpeg/libavutil/ppc/util_altivec.h
+@@ -54,6 +54,15 @@
+ #if HAVE_ALTIVEC
+ #include <altivec.h>
+ 
++/* starting with gcc 8.3, the vector keyword no longer allows usage with
++ * typedefs (e.g. vector uint32_t), only with their respective basic types
++ * (e.g. vector unsigned int) - with __vector this still works and there
++ * is code relying on it
++ */
++#ifndef vector
++#define vector __vector
++#endif
++
+ // used to build registers permutation vectors (vcprm)
+ // the 's' are for words in the _s_econd vector
+ #define WORD_0 0x00,0x01,0x02,0x03