sokol: allow submodules to be built by themselves (add required imports)

vlib/sokol/f/f.v

@@ -1,9 +1,10 @@
 module f
 
 import fontstash
+import sokol.c
 
 pub const (
-  used_import = 1 + fontstash.used_import
+  used_import = fontstash.used_import + c.used_import
 )
 
 /*

vlib/sokol/gfx/gfx.v

@@ -1,7 +1,10 @@
 module gfx
 
+import sokol.c
+
 pub const (
 	version = 1
+	used_import = c.used_import
 )
 
 // setup and misc functions

vlib/sokol/sapp/sapp.v

@@ -1,5 +1,11 @@
 module sapp
 
+import sokol.gfx
+
+pub const (
+  used_import = gfx.used_import
+)
+
 // Android needs a global reference to `g_desc`
 __global ( g_desc C.sapp_desc )