ceph.spec.in: %enable_devtoolset11 only if the macro is defined

ceph.spec.in: %enable_devtoolset11 only if the macro is defined

there is chance that we are using `yum-builddep` to prepare the
build dependencies. in that case, gcc-toolset-11-build is not
installed. it's like a chicken-egg dilemma, but the point is
`yum-builddep` is able to pull in the gcc-toolset-11-build. once
gcc-toolset-11-build is installed, we will have the %enable_devtoolset11
rpm macro.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit 75dd92353381c6ec5ac7251585a276059b8a2253)