libm: Include float.h to get LDBL_MANT_DIG

The long double aliases of double functions are only exposed as aliases if
LDBL_MANT_DIG is 53 (same as DBL_MANT_DIG). Without float.h included these
files were not exposing weak aliases as expected, leading to link failures
if programs use the *l functions. This should fix editors/calligra on
targets with 64-bit long double, which uses erfl and erfcl. Found on
powerpc64.

Reviewed by: kargl@