Add #if TARGET_OS_WASI to _CFReallocf definition (swiftlang#3026)

This allows `_CFReallocf` function to be built for Wasm/WASI.

Author: MaxDesiatov

CoreFoundation/Base.subproj/CFPlatform.c

@@ -1711,7 +1711,7 @@ int _CFOpenFile(const char *path, int opts) {
 }
 
 CF_CROSS_PLATFORM_EXPORT void *_CFReallocf(void *ptr, size_t size) {
-#if TARGET_OS_WIN32 || TARGET_OS_LINUX || defined(__OpenBSD__)
+#if TARGET_OS_WIN32 || TARGET_OS_LINUX || TARGET_OS_WASI || defined(__OpenBSD__)
     void *mem = realloc(ptr, size);
     if (mem == NULL && ptr != NULL && size != 0) {
         free(ptr);