package/sdl_mixer: disable parallel build

Build sometimes fails on:
Fatal error: can't create build/load_aiff.o: No such file or directory

Fixes:
 - http://autobuild.buildroot.org/results/c800ef60d8af0cd76f2f1de9aff573120ebd8ada

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/sdl_mixer/sdl_mixer.mk

@@ -10,6 +10,9 @@ SDL_MIXER_SITE = http://www.libsdl.org/projects/SDL_mixer/release
 SDL_MIXER_LICENSE = Zlib
 SDL_MIXER_LICENSE_FILES = COPYING
 
+# Package does not build in parallel due to improper make rules
+SDL_MIXER_MAKE = $(MAKE1)
+
 SDL_MIXER_INSTALL_STAGING = YES
 SDL_MIXER_DEPENDENCIES = sdl