Add an include guard in agi_pre.h, for whatever reason GCC likes to include the precompiled header twice. I've added a note that this needs to be looked at eventually.

Originally committed to SVN as r3531.

aegisub/src/agi_pre.h

@@ -40,8 +40,12 @@
 /// @note Make sure that you disable use of precompiled headers on md5.c and
 ///       MatroskaParser.c, as well as any possible future .c files.
 
+#ifndef AGI_PRE_H
+
+/// @brief Inclusion guard.
+/// @todo Why is this even nessicary? GCC seems to include agi_pre.h twice for no reason.
+#define AGI_PRE_H
 
-////////////
 // C++ only
 #ifdef __cplusplus
 
@@ -240,3 +244,5 @@
 #include "include/aegisub/exception.h"
 
 #endif // C++
+
+#endif // AGI_PRE_H