mjpegtools: fix build on powerpc without altivec

mpjegtools fails to build on powerpc without altivec:

build_sub44_mests.c: In function 'build_sub44_mests_altivec':
build_sub44_mests.c:268:9: internal compiler error: Segmentation fault
     vr1 = vec_ld(rowstride, (unsigned char*)s44blk);

It seems mpjegtools is wrongly detecting altivec support:
configure:   - PowerPC Optimizations:
configure:     - AltiVec enabled             : true

Fix this by adding BR2_PACKAGE_MJPEGTOOLS_SIMD_SUPPORT and setting
--enable-simd-accel / --disable-simd-accel

Fixes:
 - http://autobuild.buildroot.net/results/c9464712f43efb8954fd2e5460126ad193660353



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>