LibC: Rename FP_ILOGNAN => FP_ILOGBNAN

FP_ILOGBNAN is the correct POSIX-specified name.

Userland/Libraries/LibC/math.cpp

@@ -182,7 +182,7 @@ static int internal_ilogb(FloatT x) NOEXCEPT
         return FP_ILOGB0;
 
     if (isnan(x))
-        return FP_ILOGNAN;
+        return FP_ILOGBNAN;
 
     if (!isfinite(x))
         return INT_MAX;

Userland/Libraries/LibC/math.h

@@ -94,7 +94,7 @@ __BEGIN_DECLS
 #define DOUBLE_MIN ((double)0b0000000000010000000000000000000000000000000000000000000000000000)
 
 #define FP_ILOGB0 INT_MIN
-#define FP_ILOGNAN INT_MAX
+#define FP_ILOGBNAN INT_MAX
 
 #define FLT_EVAL_METHOD __FLT_EVAL_METHOD__