main: Fix redeclaration linking error

Fix linking error as the "store" global variable gets redeclared in
each C file that includes the header. Move the actual declaration to
main.c.

Fixes:
 /usr/bin/ld: ./.libs/libfprintd.a(device.o):/builds/libfprint/fprintd/src/storage.h:51: multiple definition of `store'; main.o:/builds/libfprint/fprintd/src/storage.h:51: first defined here

src/storage.h

@@ -46,4 +46,4 @@ struct storage {
 typedef struct storage fp_storage;
 
 /* The currently setup store */
-fp_storage store;
+extern fp_storage store;