package/systemd: add gcc >= 5.x dependency

package/systemd: add gcc >= 5.x dependency

As reported on the mailing list, there is a build issue with systemd 245
when using gcc < 5.0:

http://lists.busybox.net/pipermail/buildroot/2020-April/278931.html

Build issue:
../src/shared/gpt.c:7:9: error: initializer element is not constant
         { GPT_ROOT_X86,              "root-x86"              },

When testing with a toolchain using gcc 5.4.0 and the build is ok.
http://toolchains.bootlin.com/downloads/releases/toolchains/armv5-eabi--glibc--stable-2017.05-toolchains-1-1.tar.bz2

While searching for "error: initializer element is not constant" message, we
can notice a note about gcc 5 change about "Initializing statics with compound
literals":

https://gcc.gnu.org/gcc-5/porting_to.html

Add a dependency on gcc 5 to avoid using to old compiler.

There is the same issue with host-systemd with host gcc 4.9
(tested with Debian Jessie). So, add a dependency on host gcc >= 5.x.

Fixes:
http://autobuild.buildroot.org/results/520/520dab2253f4cbe408a8177a6587dcb38c6ba215
http://autobuild.buildroot.org/results/e0e/e0e0512de822864d670b5d176798a24ab09eed2d
http://autobuild.buildroot.org/results/f56/f5660b2711627fcee4086e096e4ec4d9ba190ab6

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>