package/mesa3d: fix musl build error

package/mesa3d: fix musl build error

To reproduce the build error use this defconfig:

BR2_x86_64=y
BR2_x86_atom=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-x86-64-musl-2017.02-744-g40962a0.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_5=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_10=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM_MUSL=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_NOUVEAU=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_R600=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_SVGA=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_SWRAST=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_VIRGL=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_SWRAST=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_I915=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_I965=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_NOUVEAU=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_RADEON=y
BR2_PACKAGE_MESA3D_OSMESA=y
BR2_PACKAGE_MESA3D_OPENGL_ES=y
BR2_PACKAGE_MESA3D_OPENGL_TEXTURE_FLOAT=y
BR2_PACKAGE_XORG7=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>