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/SDL3/SDL_platform_defines.h

@@ -183,7 +183,7 @@
 #if defined(SDL_PLATFORM_WINGDK) || defined(SDL_PLATFORM_XBOXONE) || defined(SDL_PLATFORM_XBOXSERIES)
 #define SDL_PLATFORM_GDK    1
 #endif
-#ifdef __PSP__
+#if defined(__PSP__) || defined(__psp__)
 #define SDL_PLATFORM_PSP    1
 #endif
 #if defined(__PS2__) || defined(PS2)