file_sys: Replace inclusions with forward declarations where applicable

Same behavior, minus unnecessary inclusions where not necessary.

src/core/file_sys/kernel_executable.cpp

@@ -2,9 +2,12 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#include <cstring>
+
 #include "common/string_util.h"
 #include "core/file_sys/kernel_executable.h"
 #include "core/file_sys/vfs_offset.h"
+#include "core/loader/loader.h"
 
 namespace FileSys {