Fix sysdeps/mips/__longjmp.c warning.

Fix sysdeps/mips/__longjmp.c warning.

This patch fixes

../sysdeps/mips/__longjmp.c:27:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]

which arose I think from the MIPS16 changes (renaming the function to
____longjmp with an alias __longjmp, so a prior header prototype for
__longjmp no longer sufficed to prevent a warning).  I've made the
function use a prototype definition, which is what we want for all
function definitions in glibc anyway.

Tested for MIPS.

	* sysdeps/mips/__longjmp.c (____longjmp): Use prototype
	definition.