modernize basic shader a tiny bit

"varying" is redundant here, all GS inputs & outputs vary.

Any code that uses this will be GLSL 1.3 or newer.

source/blender/gpu/shaders/gpu_shader_basic_geom.glsl

@@ -14,9 +14,9 @@ layout(line_strip, max_vertices = 10) out;
 layout(triangle_strip, max_vertices = 6) out;
 #endif
 
-varying out float t;
-varying in vec4 varying_vertex_color_line[];
-varying out vec4 varying_vertex_color;
+out float t;
+in vec4 varying_vertex_color_line[];
+out vec4 varying_vertex_color;
 
 uniform ivec4 viewport;
 uniform float line_width;