MIPS: Fix circular definition of __LDBL_MANT_DIG__ in ieee754.h

In commit aa706e13,
sysdeps/mips/ieee754/ieee754.h was changed to use GCC’s predefined
macro __LDBL_MANT_DIG__, instead of including <float.h> and using
LDBL_MANT_DIG (and therefore polluting the user namespace with all of
the macros defined in float.h).  In order to support compilers that
don’t provide __LDBL_MANT_DIG__, there is a fallback #if block which
was supposed to include <float.h> and then define __LDBL_MANT_DIG__ to
LDBL_MANT_DIG.  However, at some point during the development of the
patch, a typo was introduced, causing the fallback block to define
__LDBL_MANT_DIG__ to expand to __LDBL_MANT_DIG__.

Correct this typo.