x86/purgatory: Fix sparse warning, symbol not declared

x86/purgatory: Fix sparse warning, symbol not declared

Sparse emits warning, 'symbol not declared' for a function that has
neither file scope nor a forward declaration. The functions only call
site is an ASM file.

Add a header file with the function declaration. Include the header file in
the C source file defining the function in order to fix the sparse
warning. Include the header file in ASM file containing the call site to
document the usage.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Link: http://lkml.kernel.org/r/1487545956-2547-3-git-send-email-me@tobin.cc
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>