Redefine XINPUT_CAPABILITIES_EX as an SDL typedef

mingw-w64 has added this from Proton (which added this from SDL), so we need to re-define it as a local symbol to avoid conflicting with mingw-w64 headers.

Fixes https://github.com/libsdl-org/SDL/issues/9031

src/core/windows/SDL_xinput.h

@@ -212,7 +212,7 @@ typedef struct
 #endif /* HAVE_XINPUT_H */
 
 /* This struct is not defined in XInput headers. */
-typedef struct _XINPUT_CAPABILITIES_EX
+typedef struct
 {
     XINPUT_CAPABILITIES Capabilities;
     WORD VendorId;
@@ -220,7 +220,7 @@ typedef struct _XINPUT_CAPABILITIES_EX
     WORD ProductVersion;
     WORD unk1;
     DWORD unk2;
-} XINPUT_CAPABILITIES_EX, *PXINPUT_CAPABILITIES_EX;
+} SDL_XINPUT_CAPABILITIES_EX;
 
 /* Forward decl's for XInput API's we load dynamically and use if available */
 typedef DWORD(WINAPI *XInputGetState_t)(
@@ -244,7 +244,7 @@ typedef DWORD(WINAPI *XInputGetCapabilitiesEx_t)(
     DWORD dwReserved,                       /* [in] Must be 1 */
     DWORD dwUserIndex,                      /* [in] Index of the gamer associated with the device */
     DWORD dwFlags,                          /* [in] Input flags that identify the device type */
-    XINPUT_CAPABILITIES_EX *pCapabilitiesEx /* [out] Receives the capabilities */
+    SDL_XINPUT_CAPABILITIES_EX *pCapabilitiesEx /* [out] Receives the capabilities */
 );
 
 typedef DWORD(WINAPI *XInputGetBatteryInformation_t)(

src/joystick/windows/SDL_xinputjoystick.c

@@ -94,7 +94,7 @@ static const char *GetXInputName(const Uint8 userid, BYTE SubType)
 
 static SDL_bool GetXInputDeviceInfo(Uint8 userid, Uint16 *pVID, Uint16 *pPID, Uint16 *pVersion)
 {
-    XINPUT_CAPABILITIES_EX capabilities;
+    SDL_XINPUT_CAPABILITIES_EX capabilities;
 
     if (!XINPUTGETCAPABILITIESEX || XINPUTGETCAPABILITIESEX(1, userid, 0, &capabilities) != ERROR_SUCCESS) {
         return SDL_FALSE;
@@ -120,7 +120,7 @@ static SDL_bool GetXInputDeviceInfo(Uint8 userid, Uint16 *pVID, Uint16 *pPID, Ui
 
 int SDL_XINPUT_GetSteamVirtualGamepadSlot(Uint8 userid)
 {
-    XINPUT_CAPABILITIES_EX capabilities;
+    SDL_XINPUT_CAPABILITIES_EX capabilities;
 
     if (XINPUTGETCAPABILITIESEX &&
         XINPUTGETCAPABILITIESEX(1, userid, 0, &capabilities) == ERROR_SUCCESS &&