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@

lib/msun/src/s_cbrt.c

@@ -15,6 +15,7 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <float.h>
 #include "math.h"
 #include "math_private.h"
 

lib/msun/src/s_cproj.c

@@ -30,6 +30,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <complex.h>
+#include <float.h>
 #include <math.h>
 
 #include "math_private.h"

lib/msun/src/s_erf.c

@@ -107,7 +107,7 @@ __FBSDID("$FreeBSD$");
  *	   	erfc/erf(NaN) is NaN
  */
 
-
+#include <float.h>
 #include "math.h"
 #include "math_private.h"