psp: Fixed building with pspdev when not using CMake.

(psp-cmake defines `__PSP__` on the command line, but the compiler itself
only defines `__psp__` and some variations.

Fixes #9378.

include/SDL_platform.h

@@ -199,8 +199,10 @@
 #undef __GDK__
 #define __GDK__ 1
 #endif
-#if defined(__PSP__)
+#if defined(__PSP__) || defined(__psp__)
+#ifdef __PSP__
 #undef __PSP__
+#endif
 #define __PSP__ 1
 #endif
 #if defined(PS2)