Fix unused parameter warning.

Remove the unused `disp` argument from Mem::resetDisplacement().
Fixes the following harmless compiler warning:

    src/asmjit/x86/../x86/x86defs.h:3106:22: warning:
    unused parameter 'disp' [-Wunused-parameter]
       ASMJIT_INLINE Mem& resetDisplacement(int32_t disp) {
Merge pull request #31 from bnoordhuis/fix-compiler-warning

Fix unused parameter warning by fixing function prototype.