package/gauche: fix musl build issue

package/gauche: fix musl build issue

Add a patch from upstream to fix a musl build issue:

  In file included from os_dep.c:44:0:
  /home/peko/autobuild/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/asm/sigcontext.h:9:8: error: redefinition of 'struct sigcontext'
   struct sigcontext {
          ^
  In file included from /home/peko/autobuild/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/signal.h:243:0,
                   from ./include/private/../gc_pthread_redirects.h:42,
                   from ./include/private/../gc.h:1443,
                   from ./include/private/gc_priv.h:46,
                   from os_dep.c:17:
  /home/peko/autobuild/instance-0/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/bits/signal.h:11:16: note: originally defined here
   typedef struct sigcontext

The Linux headers sigcontext and asm/sigcontext are no longer used and were
replaced with ucontext_t in GC 7.0alpha1 (gauche uses a private copy of bdwgc).

Fixes:
http://autobuild.buildroot.net/results/411/4111ea586143410649b78c8dfc737f0f83d78441/
http://autobuild.buildroot.net/results/b19/b19dbcc7ddf0f24472afcecf4c684915045550bf/
http://autobuild.buildroot.net/results/9eb/9ebf7ad0d721433dcc8523a55d08ca074d037e58/
http://autobuild.buildroot.net/results/223/22328dbe8ea86c59ceff3ba40da1d9ef39e10533/
and many more.

Fetch from:
https://github.com/shirok/Gauche/commit/13a196557848f22a1607a300643131345e9f32b3

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>