fix "warning: old-style function definition"

This warning occurred when compiling using

gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)

on CentOS 6.5 x86_64

using the following CFLAGS (locally inserted into Makefile)

CFLAGS = -g -Wall -Wextra -Wformat=2 -funsigned-char -fstrict-aliasing
 -Wstrict-aliasing -Wfloat-equal -Wundef -Wuninitialized -Wpointer-arith
 -Wbad-function-cast -Wcast-qual -Wcast-align -Wwrite-strings -Waddress
 -Waggregate-return -Wstrict-prototypes -Wold-style-declaration
 -Wold-style-definition -Wmissing-parameter-type -Wmissing-prototypes
 -Wmissing-declarations -Wmissing-field-initializers -Wmissing-noreturn
 -Wmissing-format-attribute -Wpacked -Wredundant-decls -Wnested-externs
 -Wshadow -Wsign-compare -Wlogical-op -std=c99

and this make command:

make clean ; make example tests

example.c

@@ -12,7 +12,7 @@
 
 #include "minmea.h"
 
-int main()
+int main(void)
 {
     char line[MINMEA_MAX_LENGTH];
     while (fgets(line, sizeof(line), stdin) != NULL) {

tests.c

@@ -754,7 +754,7 @@ Suite *minmea_suite(void)
     return s;
 }
 
-int main()
+int main(void)
 {
     int number_failed;
     Suite *s = minmea_suite();