Fix collision of exp declared by mathcalls.h is avoided.

main.c: In function 'process_connection':
main.c:219: error: declaration of 'exp' shadows a global declaration
/usr/include/bits/mathcalls.h:101: error: shadowed declaration is here
make[2]: * [boothd-main.o] Error 1
make[2]: Leaving directory `/root/cluster-packages/y.booth/src'
make[1]: * [all] Error 2
make[1]: Leaving directory `/root/cluster-packages/y.booth/src'
make: *** [all-recursive] Error 1