fix build on gcc-10 (-fno-common)

[trofi]

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

As a result build fails as:

ld: i3lock-unlock_indicator.o:(.bss+0xc): multiple definition of
  `input_position'; i3lock-i3lock.o:(.bss+0x3c): first defined here

The change turned one of definitions into declaration.