Fix exports for shared libraries

We use "shared" to define a shared library and not dll.

Change-Id: Ia97ebd0042a7ef0f33eadaa448d9a44b42331ad1
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/libs/cplusplus/cplusplus-lib.pri

@@ -1,4 +1,4 @@
-contains(CONFIG, dll) {
+shared {
     DEFINES += CPLUSPLUS_BUILD_LIB
 } else {
     DEFINES += CPLUSPLUS_BUILD_STATIC_LIB