headers/kernel: Move B_WAIT_TILL_LOADED to a syscall header.

It is only used as an argument to _kern_load_image directly, not to
any of the load_image functions in image.h, so it belongs in a syscall-
specific header like other such constants.

No functional change intended.

src/bin/debug/debug_utils.cpp

@@ -17,6 +17,7 @@
 
 #include <libroot_private.h>
 #include <syscalls.h>
+#include <syscall_load_image.h>
 
 
 extern const char* __progname;