[klibc] arm: use rt signals

Fixes arm* breakage.

Tested-by: Thorsten Glaser <tg@mirbsd.org>
Signed-off-by: maximilian attems <max@stro.at>
diff --git a/usr/include/arch/arm/klibc/archconfig.h b/usr/include/arch/arm/klibc/archconfig.h
index 36e9ab5..cfe847a 100644
--- a/usr/include/arch/arm/klibc/archconfig.h
+++ b/usr/include/arch/arm/klibc/archconfig.h
@@ -15,4 +15,7 @@
 # define _KLIBC_ARM_USE_BX 1
 #endif
 
+/* Use rt_* signals */
+#define _KLIBC_USE_RT_SIG 1
+
 #endif				/* _KLIBC_ARCHCONFIG_H */