gravit: 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: tool.o:(.bss+0x0): multiple definition of `view'; color.o:(.bss+0x0): first defined here
ld: tool.o:(.bss+0x820): multiple definition of `video'; color.o:(.bss+0x820): first defined here
ld: png_save.o:(.bss+0x0): multiple definition of `view'; color.o:(.bss+0x0): first defined here

The change leaves variable definitions only in C files.

[gak]

Hi! Thanks for the PR. I'm not maintaining this project any more, but I've merged it in anyway!