keeper.c: include stdint.h for uintptr_t typedef

Corrects this build error:

	gcc -Wall -Werror -O2 -I/usr/include/lua5.1  -D_GNU_SOURCE -fPIC   -c -o keeper.o keeper.c
	keeper.c: In function ‘keeper_acquire’:
	keeper.c:626:21: error: ‘uintptr_t’ undeclared (first use in this function)
	   unsigned int i= ((uintptr_t)(ptr) >> 3) % GNbKeepers;
	                     ^
	keeper.c:626:21: note: each undeclared identifier is reported only once for each function it appears in
	<builtin>: recipe for target 'keeper.o' failed
	make[1]: *** [keeper.o] Error 1

Signed-off-by: Steven Noonan <steven@uplinklabs.net>