Undefined symbols for architecture armv7

[kpkumarkpk]

I added URBAlertView.h and URBAlertView.m at first working fine after two days again I checked automatically I got these errors.

Undefined symbols for architecture armv7:
"_va_start", referenced from:
-[URBAlertView initWithTitle:message:cancelButtonTitle:otherButtonTitles:] in URBAlertView.o
ld: symbol(s) not found for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)