Previously bitset.h would include u_math.h to get bitscan.h.  u_math.h
lives in src/gallium/auxiliary/util while both bitset.h and bitscan.h
live in src/util.  Having the one file directly include another file
that lives in the same directory makes much more sense.

As a side-effect, several files need to directly include standard header
files that were previously indirectly included.

v2: Fix build break in src/amd/common/ac_nir_to_llvm.c.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>