LibC: Define ULLONG_MAX

Some ports require this constant to be defined as it is specified in
the standard[1].

[1] https://www.cplusplus.com/reference/climits/

Userland/Libraries/LibC/limits.h

@@ -52,6 +52,7 @@
 #define LLONG_MIN LONG_LONG_MIN
 
 #define ULONG_LONG_MAX 18446744073709551615ULL
+#define ULLONG_MAX ULONG_LONG_MAX
 
 #define CHAR_MIN SCHAR_MIN
 #define CHAR_MAX SCHAR_MAX