libsigsegv: disable for xtensa

The xtensa uClibc port does not provide the ucontext.h context manipulation
routines that libsigsegv needs.

Fixes:
http://autobuild.buildroot.net/results/06d/06d091f2ea8d08a900dd7304bab5eeeeb85f7353/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libsigsegv/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_LIBSIGSEGV
 	bool "libsigsegv"
+	depends on !BR2_xtensa
 	help
 	  Library for handling page faults in user mode.