osx: fix inconsistent use of the macro TARGET_OS_OSX

Fixed the build error on iOS:

```
httplib.h:3583:3: error: unknown type name 'CFStringRef'
  870 |   CFStringRef hostname_ref = CFStringCreateWithCString(
```

Note, `TARGET_OS_OSX` is defined but is 0 when `TARGET_OS_IOS` is 1,
and vise versa. Hence, `TARGET_OS_MAC` should have been used, that is
set to 1 for the both targets.

Author: sergio-nsk

httplib.h

@@ -3578,7 +3578,7 @@ inline int getaddrinfo_with_timeout(const char *node, const char *service,
   }
 
   return ret;
-#elif defined(TARGET_OS_OSX)
+#elif TARGET_OS_OSX
   // macOS implementation using CFHost API for asynchronous DNS resolution
   CFStringRef hostname_ref = CFStringCreateWithCString(
       kCFAllocatorDefault, node, kCFStringEncodingUTF8);
@@ -6258,8 +6258,7 @@ inline bool load_system_certs_on_windows(X509_STORE *store) {
 
   return result;
 }
-#elif defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN) &&                    \
-    defined(TARGET_OS_OSX)
+#elif defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN) && TARGET_OS_OSX
 template <typename T>
 using CFObjectPtr =
     std::unique_ptr<typename std::remove_pointer<T>::type, void (*)(CFTypeRef)>;
@@ -11008,8 +11007,7 @@ inline bool SSLClient::load_certs() {
 #ifdef _WIN32
       loaded =
           detail::load_system_certs_on_windows(SSL_CTX_get_cert_store(ctx_));
-#elif defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN) &&                    \
-    defined(TARGET_OS_OSX)
+#elif defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN) && TARGET_OS_OSX
       loaded = detail::load_system_certs_on_macos(SSL_CTX_get_cert_store(ctx_));
 #endif // _WIN32
       if (!loaded) { SSL_CTX_set_default_verify_paths(ctx_); }