Fix compilation on Solaris

Trying to compile R16B01 on SmartOS resulted in following errors:

    gcc -c -o ../priv/obj/i386-pc-solaris2.11/ferrule.o -Werror=return-type  -g -O2 -I/root/otp_src_R16B01/erts/i386-pc-solaris2.11  -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -fno-tree-copyrename  -DHAVE_CONFIG_H  ferrule.c
    ferrule.c:53:1: error: return type defaults to 'int' [-Werror=return-type]
    ...
    gcc -c -o ../priv/obj/i386-pc-solaris2.11/mod_syslog.o -Werror=return-type  -g -O2 -I/root/otp_src_R16B01/erts/i386-pc-solaris2.11  -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -fno-tree-copyrename  -DHAVE_CONFIG_H  mod_syslog.c
    mod_syslog.c:50:1: error: return type defaults to 'int' [-Werror=return-type]
    mod_syslog.c: In function 'make_exit':
    mod_syslog.c:136:5: warning: incompatible implicit declaration of built-in function 'exit' [enabled by default]
    mod_syslog.c: In function 'main':
    mod_syslog.c:131:1: error: control reaches end of non-void function [-Werror=return-type]

Adding return values and types to `main` functions in `mod_syslog.c` and
`ferrule.c` fixed the build.