tests: mem_protect: fix test prototypes

The implementations of the test cases had the wrong prototype.
The extern declarations (which were in a C file for some reason)
were correct.

I don't want to talk about the subtle code generation and stack
corruption issues that emerged from this which at one point made
me question my own sanity.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>