qtbase: fix 0003-Add-external-hostbindir-option.patch

The isEmpty function takes a variable name instead of a value.
As $$[QT_EXTERNAL_HOST_BINS] is a built-in we need to assign it to a
variable to check if it is empty.

If QT_EXTERNAL_HOST_BINS is empty (as is the case for generated Qt 5
toolchains), we need to fallback to QT_HOST_BINS when determining
the path for tools such as moc otherwise you get the following error
when building a Qt project using the generated toolchain:
make[1]: /moc: Command not found

Signed-off-by: Jonathan Liu <net147@gmail.com>