gl_shader_decompiler: Correct return value of WriteTexsInstruction()

This should be returning void, not a std::string

src/video_core/renderer_opengl/gl_shader_decompiler.cpp

@@ -750,7 +750,7 @@ private:
         }
     }
 
-    std::string WriteTexsInstruction(const Instruction& instr, const std::string& coord,
+    void WriteTexsInstruction(const Instruction& instr, const std::string& coord,
                               const std::string& texture) {
         // Add an extra scope and declare the texture coords inside to prevent
         // overwriting them in case they are used as outputs of the texs instruction.