Some GCC compilation fixes (#3114)

* Fix 6-byte multichar literals

* Add `struct` to `e_ga_class::ga_s`

* Fix remaining wrong forward declares (struct/class)

* Replace `#include <string>` with `#include <cstring>`

* Guard FLT_EPSILON define to prevent redefinition

* Add missing `#include <cstring>` for direct cstring function usage

src/d/actor/d_a_ni.cpp

@@ -13,6 +13,7 @@
 #include "c/c_damagereaction.h"
 #include "Z2AudioLib/Z2Instances.h"
 #include "f_op/f_op_camera_mng.h"
+#include <cstring>
 
 enum Joint {
     /* 0x0 */ JNT_WAIST,