LibC: Don't include fenv.h from float.h

This fixes a build issue where `compiler-rt` tried to declare its own
version of the `FE_*` macros, and included `float.h` in order to get the
constants. `compiler-rt` tried to declare these as an enum, and failed
with a syntax error when the constant's literal values were substituted.

Userland/Libraries/LibC/float.h

@@ -7,7 +7,8 @@
 
 #pragma once
 
-#include <fenv.h>
+// Defined in fenv.cpp, but we must not include fenv.h, so here's its prototype.
+int fgetround();
 
 #define FLT_RADIX 2
 #define DECIMAL_DIG 21