stdbit-h: Fix compilation error with MSVC in C++ mode. * lib/stdbit.in.h (_BitScanReverse, _BitScanReverse64, _BitScanForward, _BitScanForward64, __cpuid, __popcnt, __popcnt64): Declare with a prototype. * lib/count-leading-zeros.h (_BitScanReverse, _BitScanReverse64): Likewise. * lib/count-trailing-zeros.h (_BitScanForward, _BitScanForward64): Likewise. * lib/count-one-bits.h (__cpuid, __popcnt, __popcnt64): Likewise.