Guard _PSP_FW_VERSION macro to mitigate warnings with -Wundef

Fixes #123. Guards `_PSP_FW_VERSION` references with `#if defined()` to avoid the SDK headers returning warnings when compiling modules with the `-Wundef` cflag.

src/user/pspsysmem.h

@@ -102,7 +102,7 @@ SceSize sceKernelMaxFreeMemSize(void);
  */
 int sceKernelDevkitVersion(void);
 
-#if _PSP_FW_VERSION >= 150
+#if defined(_PSP_FW_VERSION) && _PSP_FW_VERSION >= 150
 
 /**
  * Kernel printf function.