glapi: add const qualifier to glShaderSourceARB() parameter

Fixes the es2 build with gcc.

Note: in glext.h the prototypes for glShaderSource() and glShaderSourceARB()
disagree:  only the former has the extra const qualifier.

Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mapi/glapi/gen/gl_API.xml

@@ -5363,7 +5363,7 @@
     <function name="ShaderSource" alias="ShaderSourceARB">
         <param name="shader" type="GLuint"/>
         <param name="count" type="GLsizei"/>
-        <param name="string" type="const GLchar **"/>
+        <param name="string" type="const GLchar * const *"/>
         <param name="length" type="const GLint *"/>
         <glx ignore="true"/>
     </function>

src/mesa/main/APIspec.xml

@@ -2946,7 +2946,7 @@
 		<return type="void"/>
 		<param name="shader" type="GLuint"/>
 		<param name="count" type="GLsizei"/>
-		<param name="string" type="const GLchar **"/>
+		<param name="string" type="const GLchar * const *"/>
 		<param name="length" type="const int *"/>
 	</proto>
 </template>