Add symbol visibility annotations to symbols in SPIRV/

Symbols in the SPIRV/ directory that are meant to be part of the public
API of glslangnow have GLSLANG_EXPORT annotations that make sure they
will be accessible when building glslang as a shared library if
-fvisibility=hidden is enabled.

SPIRV/disassemble.h

@@ -43,10 +43,12 @@
 #include <iostream>
 #include <vector>
 
+#include "glslang/Include/visibility.h"
+
 namespace spv {
 
     // disassemble with glslang custom disassembler
-    void Disassemble(std::ostream& out, const std::vector<unsigned int>&);
+    GLSLANG_EXPORT void Disassemble(std::ostream& out, const std::vector<unsigned int>&);
 
 }  // end namespace spv