[sigslots_lite] new c++14 insists on const expr init in the same translation unit

[stonier]

This fixes the errors:

23:07:06   233 |                 static sigslots::GlobalSlot<Data> slots[capacity];
23:07:06       |                                                   ^~~~~
23:07:06 /tmp/binarydeb/ros-rolling-ecl-sigslots-lite-1.1.0/src/examples/../../include/ecl/sigslots_lite/managers.hpp: In static member function ‘static ecl::lite::sigslots::GlobalSlot<void>* ecl::lite::GlobalSlots<void, Dummy>::addSlot(void (*)())’:
23:07:06 /tmp/binarydeb/ros-rolling-ecl-sigslots-lite-1.1.0/src/examples/../../include/ecl/sigslots_lite/managers.hpp:368:51: error: ISO C++ forbids variable length array ‘slots’ [-Werror=vla]
23:07:06   368 |                 static sigslots::GlobalSlot<void> slots[capacity];

Basically, c++14 with -Wpedantic insists on initialisation of const expr (for array bounds) in the same translation unit. This redesigns it so storage is more (to the tune of about 5 lines) in the hands of the user.