Do not write 'Libs:' into .pc files if TEMPLATE is not 'lib'

Especially for header modules we don't want a 'Libs:' entry in their
.pc file.

Task-number: QTBUG-75901
Change-Id: I39037d3132e39dd360532e1425f794ebec28e0bd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

qmake/generators/makefile.cpp

@@ -3359,6 +3359,7 @@ MakefileGenerator::writePkgConfigFile()
     if (!version.isEmpty())
         t << "Version: " << version << endl;
 
+    if (project->first("TEMPLATE") == "lib") {
         // libs
         t << "Libs: ";
         QString pkgConfiglibName;
@@ -3396,6 +3397,7 @@ MakefileGenerator::writePkgConfigFile()
                 t << ' ' << fixLibFlags((*it).toKey()).join(' ');
             t << endl;
         }
+    }
 
     // flags
     // ### too many