package/optee-client: add dependency on headers >= 4.3

Since upstream commit 5da4da99e0bb3d694e93046207ae4ce3b5545043, in
3.13.0, OP-TEE client unconditionally uses the MMC_IOC_MULTI_CMD
ioctl(). But this ioctl() was only introduced in kernel 4.3, so
earlier kernels did not have this definition.

This was clearly known when upstream started using this ioctl. From
the upstream commit 5da4da99e0bb3d694e93046207ae4ce3b5545043 commit
log:

    As MMC_IOC_MULTI_CMD is available since linux 4.3, we should be safe to
    use it by default on read / write block operations.

So let's propagate this dependency into Buildroot.

Fixes:

  http://autobuild.buildroot.net/results/bd270a6793df499d5f52959e3de17f8a92d88305/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>