Fix building with GCC 10 (#20)

When trying to build with gcc 10, there is a linker error:

    /usr/bin/ld: /tmp/ccjRQbpX.o:/builddir/build/BUILD/entr-4.4/data.h:33: multiple definition of `files'; /tmp/ccHpowMW.o:/builddir/build/BUILD/entr-4.4/missing/../data.h:33: first defined here
    collect2: error: ld returned 1 exit status

This is a known thing in gcc 10:

https://gcc.gnu.org/gcc-10/porting_to.html#common

This patch adds extern keyword to the header file and removes it from
exactly one source file.

data.h

@@ -29,5 +29,5 @@ typedef struct {
 	ino_t ino;
 } WatchFile;
 
-/* declare as extern in source */
-WatchFile **files;
+/* defined in entr.c */
+extern WatchFile **files;

entr.c

@@ -69,7 +69,7 @@ int (*xtcsetattr)(int fd, int action, const struct termios *tp);
 /* globals */
 
 extern int optind;
-extern WatchFile **files;
+WatchFile **files;
 WatchFile *leading_edge;
 int child_pid;