Add missing 'else's to fix extension processing

The missing else-statements caused all of the extensions execpt
GL_ARB_texture_rectangle to be unsupported.

This causes the following tests to pass:

     glslparsertest/glsl2/draw_buffers-04.frag
diff --git a/glsl_parser_extras.cpp b/glsl_parser_extras.cpp
index f7ee891..1f74cbb 100644
--- a/glsl_parser_extras.cpp
+++ b/glsl_parser_extras.cpp
@@ -129,7 +129,7 @@
 			  ? "enable" : "require");
 	 return false;
       }
-   } if (strcmp(name, "GL_ARB_draw_buffers") == 0) {
+   } else if (strcmp(name, "GL_ARB_draw_buffers") == 0) {
       /* This extension is only supported in fragment shaders.
        */
       if (state->target != fragment_shader) {
@@ -138,7 +138,7 @@
 	 state->ARB_draw_buffers_enable = (ext_mode != extension_disable);
 	 state->ARB_draw_buffers_warn = (ext_mode == extension_warn);
       }
-   } if (strcmp(name, "GL_ARB_texture_rectangle") == 0) {
+   } else if (strcmp(name, "GL_ARB_texture_rectangle") == 0) {
       state->ARB_texture_rectangle_enable = (ext_mode != extension_disable);
       state->ARB_texture_rectangle_warn = (ext_mode == extension_warn);
    } else {