Since 4.11 signal_pending is declared in linux/sched/signal.h. Using
linux/sched.h result in a build failure :

spec-sw/kernel/wr-nic-dio.c: In function 'wrn_dio_cmd_stamp':
spec-sw/kernel/wr-nic-dio.c:267:7: error: implicit declaration of function 'signal_pending'; did you mean 'timer_pending'? [-Werror=implicit-function-declaration]
   if (signal_pending(current))
       ^~~~~~~~~~~~~~
       timer_pending
cc1: some warnings being treated as errors
make[5]: *** [/usr/src/linux-headers-4.18.0-2-common/scripts/Makefile.build:323: spec-sw/kernel/wr-nic-dio.o] Error 1
make[4]: *** [/usr/src/linux-headers-4.18.0-2-common/Makefile:1531: _module_spec-sw/kernel] Error 2
make[3]: *** [Makefile:146: sub-make] Error 2
make[2]: *** [Makefile:8: all] Error 2
make[2]: Leaving directory '/usr/src/linux-headers-4.18.0-2-amd64'
make[1]: *** [Makefile:20: all] Error 2
make[1]: Leaving directory 'spec-sw/kernel'
make: *** [Makefile:28: kernel] Error 2

This patch adds preprocessor to include correct head according to the kernel version.
Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>