qbs project files: Qualify access to product properties in Export items.

Such items behave "module-like" and one should therefore not count on
them having access to the scope of their parent item.

Change-Id: I8a323ae31218c6b1ee721ffaa65c04ccf08943bc
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

qbs/imports/QtcLibrary.qbs

@@ -27,6 +27,6 @@ QtcProduct {
 
     Export {
         Depends { name: "cpp" }
-        cpp.includePaths: [libIncludeBase]
+        cpp.includePaths: [product.libIncludeBase]
     }
 }

qbs/imports/QtcPlugin.qbs

@@ -52,6 +52,6 @@ QtcProduct {
     Export {
         Depends { name: "ExtensionSystem" }
         Depends { name: "cpp" }
-        cpp.includePaths: [pluginIncludeBase]
+        cpp.includePaths: [product.pluginIncludeBase]
     }
 }