alsa-lib: put BR2_PACKAGE_ALSA_LIB_PYTHON under the existing if condition

Instead of duplicating the "depends on BR2_PACKAGE_ALSA_LIB", just put
BR2_PACKAGE_ALSA_LIB_PYTHON under the existing condition "if
BR2_PACKAGE_ALSA_LIB".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>