loader: Add volatile to resource_being_loaded.

Some compiler optimizers seem to assume that dlopen will not use
__attribute__((constructor)) functions to call back to the program.
This was causing resource_being_loaded to be optimized away completely.

ASTERISK-27531 #close
Tested By: abelbeck

Change-Id: If17a3b889e06811a0e7119f0539d052494d6ece9