devel/qt6-tools: Enable build with ccache after [1]

We don't have to build with a particular llvm version, but need to
specificy an llvm version to link against. This restores build behavior
before [1].

[1] https://cgit.freebsd.org/ports/commit/?id=4df0e66a3662ac9d427c2dcd79eb8555236083dd

devel/qt6-tools/Makefile

@@ -10,7 +10,7 @@ BUILD_DEPENDS=	${LOCALBASE}/include/vulkan/vulkan.h:graphics/vulkan-headers
 LIB_DEPENDS=	libxkbcommon.so:x11/libxkbcommon \
 		libzstd.so:archivers/zstd
 
-USES=		cmake compiler:c++17-lang gl llvm:lib python:build \
+USES=		cmake compiler:c++17-lang gl llvm:lib,noexport python:build \
 		qt-dist:6
 USE_GL=		opengl
 USE_QT=		base declarative