glabels: fix build failure 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: label.o:src/template-history.h:31: multiple definition of `gl_template_history'; glabels-batch.o:src/template-history.h:31: first defined here
ld: label-text.o:src/font-history.h:31: multiple definition of `gl_font_history'; glabels-batch.o:src/font-history.h:31: first defined here
ld: font-history.o:src/font-history.h:31: multiple definition of `gl_font_history'; glabels-batch.o:src/font-history.h:31: first defined here
ld: template-history.o:src/template-history.h:31: multiple definition of `gl_template_history'; glabels-batch.o:src/template-history.h:31: first defined here

The change drops redundant definitions in headers and relies on
definitions in .c files.