Suppress prototype warning

[take-cheeze]

clang -O3 --std=c99 -pedantic -Wall xkbcat.c -o xkbcat -lX11 -lXi
xkbcat.c:15:15: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
int printUsage() {
              ^
               void
1 warning generated.