qemu: do not #include <sys/signal.h>

For our LIBCs including sys/signal.h is redundant.
For glibc sys/signal.h has just one line: #include <signal.h>
For musl libc it prints an annoying warning.

srcpkgs/qemu/patches/ignore-sys-signal_h.patch

@@ -0,0 +1,17 @@
+Source: @pullmoll
+Upstream: no
+Reason: Not needed. For glibc this has just #include <signal.h> and for musl it prints a warning.
+
+--- osdep.h	2020-12-08 17:59:44.000000000 +0100
++++ /tmp/void/glibc-2/builddir/qemu-5.2.0/include/qemu/osdep.h	2020-12-12 18:41:47.709685865 +0100
+@@ -104,10 +104,6 @@
+ #include <setjmp.h>
+ #include <signal.h>
+ 
+-#ifdef HAVE_SYS_SIGNAL_H
+-#include <sys/signal.h>
+-#endif
+-
+ #ifndef _WIN32
+ #include <sys/wait.h>
+ #else