lm4f: Fix include guard in nvic.h

The lm4f/nvic.h include guard was wrongfully named LIBOPENCM3_LM3S_NVIC_H.
This caused the lm3s/nvic.h include a few lines down to not be compiled.
Rename the inlcude guard to the more appropriate LIBOPENCM3_LM4F_NVIC_H.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>