[libc] Fix size_t used without including stddef.h in CPP/limit.h. (#78861)

Author: lntue

libc/src/__support/CPP/limits.h

@@ -20,7 +20,7 @@ namespace cpp {
 
 // Some older gcc distributions don't define these for 32 bit targets.
 #ifndef LLONG_MAX
-constexpr size_t LLONG_BIT_WIDTH = sizeof(long long) * 8;
+constexpr unsigned int LLONG_BIT_WIDTH = sizeof(long long) * 8;
 constexpr long long LLONG_MAX = ~0LL ^ (1LL << (LLONG_BIT_WIDTH - 1));
 constexpr long long LLONG_MIN = 1LL << (LLONG_BIT_WIDTH - 1);
 constexpr unsigned long long ULLONG_MAX = ~0ULL;