Fix -Werror,-Wmacro-redefined in limits.h

Those macros are also defined by the compiler-provided float.h which
will be included later on (at least in the FreeBSD buildworld case) and
triggers these -Werror warnings. Including <float.h> first and only
defining the macros when DBL_DIG/FLT_DIG is missing fixes this problem.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Closes #10864

lib/libspl/include/limits.h

@@ -25,16 +25,21 @@
  */
 
 #include_next <limits.h>
+#include <float.h>
 
 #ifndef _LIBSPL_LIMITS_H
 #define	_LIBSPL_LIMITS_H
 
+#ifndef DBL_DIG
 #define	DBL_DIG		15
 #define	DBL_MAX		1.7976931348623157081452E+308
 #define	DBL_MIN		2.2250738585072013830903E-308
+#endif
 
+#ifndef FLT_DIG
 #define	FLT_DIG		6
 #define	FLT_MAX		3.4028234663852885981170E+38F
 #define	FLT_MIN		1.1754943508222875079688E-38F
+#endif
 
 #endif /* _LIBSPL_LIMITS_H */