CMake: Fix SBOM configuration error with CMake 3.16

SBOM generation attempts to access the SOURCE_DIR property of
interface libraries to derive a file source location.
This causes a configure error when using CMake 3.16 which doesn't
allow accessing properties not starting with INTERFACE_ on interface
libraries.

Interface libraries don't have a source dir anyway, so skip extracting
the directory for interface libraries to avoid the error.

Pick-to: 6.10 6.8
Change-Id: If70da1908dee9aba06da37aff5b0d12633a7623a
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from commit 72a47e1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

Author: alcroito

cmake/QtPublicSbomQtEntityHelpers.cmake

@@ -288,6 +288,13 @@ function(_qt_internal_sbom_get_qt_entity_repo_source_dir target)
         message(FATAL_ERROR "OUT_VAR must be set")
     endif()
 
+    # Interface libs don't have a source dir. Return early, to avoid errors with CMake 3.16.
+    get_target_property(target_type "${target}" TYPE)
+    if(target_type STREQUAL "INTERFACE_LIBRARY")
+        set(${arg_OUT_VAR} "" PARENT_SCOPE)
+        return()
+    endif()
+
     get_target_property(repo_source_dir "${target}" SOURCE_DIR)
 
     # Get the path relative to the PROJECT_SOURCE_DIR