fix library retrieval in pkg-config source, take 2

the entry's libs field is supposed to be a single pre-quoted string.
that implies that the libs and libpaths need to be joined first.

amends 7e298e2f.

Task-number: QTBUG-60925
Change-Id: Id85b2784e3c081b3ff8eb9ee2216976691f8580d
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/qt_configure.prf

@@ -581,7 +581,8 @@ defineTest(qtConfLibrary_pkgConfig) {
     version ~= s/[^0-9.].*$//
     $${1}.version = $$first(version)
     export($${1}.version)
-    $${1}.libs = $$libpaths $$libs
+    libpaths += $$libs
+    $${1}.libs = "$$libpaths"
     export($${1}.libs)
     return(true)
 }