#84 gerbv: fix build on gcc-10 (-fno-common)

[Sergei Trofimovich]

gcc-10 changed the default from -fcommon to fno-common:
https://gcc.gnu.org/PR85678

As a result build fails as:

ld: interface.o:/build/gerbv/src/callbacks.h:50: multiple definition of
  `toggle_layer'; callbacks.o:/build/gerbv/src/callbacks.h:50: first defined here
ld: interface.o:/build/gerbv/src/callbacks.h:44: multiple definition of
  `CALLBACKS_SAVE_FILE_TYPE'; callbacks.o:/build/gerbv/src/callbacks.h:44: first defined here
ld: render.o:/build/gerbv/src/callbacks.h:44: multiple definition of
  `CALLBACKS_SAVE_FILE_TYPE'; callbacks.o:/build/gerbv/src/callbacks.h:44: first defined here

The change drops accidental global variable declarations.