mpdecimal: fix comment about fenv.h

We're disabling fenv.h usage on i386 because it is not available in
musl/uclibc (and not musl/glibc as was written).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>