Fix duplicate symbol error by making error_message static

Use internal linkage for error_message to avoid linker collision
when both OpenAttributeGraph and OpenRenderBox are linked together.

Author: Kyle-Ye

Sources/OpenAttributeGraphCxx/Misc/assert.cpp

@@ -9,7 +9,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-char* error_message = nullptr;
+static char* error_message = nullptr;
 
 namespace OAG {
 void precondition_failure(const char *format, ...) {