strace: Define own sigmask macro if not defined already in signal.h

strace: Define own sigmask macro if not defined already in signal.h

This is required especially on uclibc systems where BSD compatible
obsolete functions are disabled by kconfig. Therefore we check
if the macro is undefined then we define it. Therefore eglibc/glibc
builds should work as they use to.

Signed-off-by: Khem Raj <raj.khem@gmail.com>