email: fix build against 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: utils.o:include/email.h:81: multiple definition of `Mopts'; email.o:include/email.h:81: first defined here
ld: utils.o:include/email.h:64: multiple definition of `table'; email.o:include/email.h:64: first defined here
ld: utils.o:include/email.h:65: multiple definition of `conf_file'; email.o:include/email.h:65: first defined here

The change moves globals definitions from include/email.h files to src/email.c.