package/mariadb: handle missing ucontext_t

package/mariadb: handle missing ucontext_t

On some archs uclibc does not provide ucontext_t, for details see
https://git.buildroot.net/buildroot/commit/?id=f1cbfeea95e6287c7a666aafc182ffa318eff262

This patch fixes a build error

In file included from /home/bernd/buildroot/output/build/mariadb-10.2.17/libmariadb/plugins/pvio/pvio_socket.c:32:0:
/home/bernd/buildroot/output/build/mariadb-10.2.17/libmariadb/include/ma_context.h:62:3: error: unknown type name ‘ucontext_t’
   ucontext_t base_context;
   ^~~~~~~~~~

using this defconfig:

BR2_arcle=y
BR2_archs38=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-arcle-hs38-full-2018.02-926-gb393c24.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_7=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_1=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_PACKAGE_MYSQL=y
BR2_PACKAGE_MARIADB=y
BR2_PACKAGE_MARIADB_SERVER=y

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