[cmath.syn] Align declaration of fpclassify with other functions

[eisenwave]

Throughout [cmath.syn], the established style is to align function names to account for differences in return type length, such as in:

double      nan(const char* tagp);
float       nanf(const char* tagp);
long double nanl(const char* tagp);

fpclassify does not follow that style, which is inconsistent.