alsa-lib: fix with with BR2_PACKAGE_ALSA_LIB_PYTHON

Tell it where to find the python headers.

package/multimedia/alsa-lib/alsa-lib.mk

@@ -23,6 +23,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_ALSA_LIB_PYTHON),y)
 ALSA_LIB_CONFIGURE_OPTS += --with-pythonlibs=-lpython$(PYTHON_VERSION_MAJOR)
+ALSA_LIB_CFLAGS+=-I$(STAGING_DIR)/usr/include/python$(PYTHON_VERSION_MAJOR)
 ALSA_LIB_DEPS = libpython
 else
 ALSA_LIB_CONFIGURE_OPTS += --disable-python