LibC+LibDl: Declare functions taking no arguments as taking void

In C++, a function declaration with an empty parameter list means that
the function takes no arguments. In C, however, it means that the
function takes an unspecified number of parameters.

What we did previously was therefore non-conforming. This caused a
config check to fail in the curl port, as it was able to redeclare
`rand` as taking an int parameter.

Userland/Libraries/LibC/float.h

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