irq_work: fix compile failure on MIPS from system.h split

Builds of the MIPS platform ip32_defconfig fails as of commit
0195c002 ("Merge tag 'split-asm_system_h ...") because MIPS xchg()
macro uses BUILD_BUG_ON and it was moved in commit b81947c6
("Disintegrate asm/system.h for MIPS").

The root cause is that the system.h split wasn't tested on a baseline
with commit 6c03438e

 ("kernel.h: doesn't explicitly use bug.h, so
don't include it.")

Since this file uses BUG code in several other places besides the xchg
call, simply make the inclusion explicit.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>