GCC 10 builds with -fno-common by default, which causes linker errors when
variables are declared in header files and included in multiple places.
See also: https://gcc.gnu.org/gcc-10/porting_to.html
Every .c file has to include the corresponding .h file first to make sure the
headers are self-contained. Additionally, this moves some unneeded includes
around.
Signed-off-by: Uli Schlachter <psychon@znc.in>