asm/sigcontext.h is required to build unwind-dw2.c in gcc-4.9.

Change-Id: Ie36b68d1260a656056f66a67750232a7da78c61f
Signed-off-by: Steve Ellcey <steve.ellcey@imgtec.com>
diff --git a/libc/include/signal.h b/libc/include/signal.h
index 0063b24..f1849c5 100644
--- a/libc/include/signal.h
+++ b/libc/include/signal.h
@@ -34,6 +34,7 @@
 #include <limits.h>		/* For LONG_BIT */
 #include <string.h>		/* For memset() */
 #include <sys/types.h>
+#include <asm/sigcontext.h>
 
 #if defined(__LP64__) || defined(__mips__)
 /* For 64-bit (and mips), the kernel's struct sigaction doesn't match the POSIX one,