The kbuild test robot reports that when building with W=1, GCC will warn
for a couple of missing prototypes in syscall.c:

|  arch/arm64/kernel/syscall.c:157:6: warning: no previous prototype for 'do_el0_svc' [-Wmissing-prototypes]
|    157 | void do_el0_svc(struct pt_regs *regs)
|        |      ^~~~~~~~~~
|  arch/arm64/kernel/syscall.c:164:6: warning: no previous prototype for 'do_el0_svc_compat' [-Wmissing-prototypes]
|    164 | void do_el0_svc_compat(struct pt_regs *regs)
|        |      ^~~~~~~~~~~~~~~~~

While this isn't a functional problem, as a general policy we should
include the prototype for functions wherever possible to catch any
accidental divergence between the prototype and implementation. Here we
can easily include <asm/exception.h>, so let's do so.

While there are a number of warnings elsewhere and some warnings enabled
under W=1 are of questionable benefit, this change helps to make the
code more robust as it evolved and reduces the noise somewhat, so it
seems worthwhile.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/202101141046.n8iPO3mw-lkp@intel.com
Link: https://lore.kernel.org/r/20210114124812.17754-1-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>