Remove extern from function declared in headers. (#13790)

By default functions are declared as extern in C/C++ headers. The goal
of this PR is to reduce the wall of text that many headers have and,
more importantly, to make the declaration of extern'd variables - of
which we have many dispersed in various places - easily and quickly
identifiable.

Automatically generated with:

    $ git grep -l '^extern.*(' '**.h' | \
            grep -v libjudy | \
            grep -v 'sqlite3.h' | \
            xargs sed -i -e 's/extern \(.*(.*$\)/\1/'

This is a NFC.

1 files changed, 5 insertions, 5 deletions

diff --git a/registry/registry_machine.h b/registry/registry_machine.h
index 77ab5aaf51..bc95ecf699 100644
--- a/registry/registry_machine.h
+++ b/registry/registry_machine.h
@@ -34,10 +34,10 @@ struct registry_machine {
 };
 typedef struct registry_machine REGISTRY_MACHINE;
 
-extern REGISTRY_MACHINE *registry_machine_find(const char *machine_guid);
-extern REGISTRY_MACHINE_URL *registry_machine_url_allocate(REGISTRY_MACHINE *m, REGISTRY_URL *u, time_t when);
-extern REGISTRY_MACHINE *registry_machine_allocate(const char *machine_guid, time_t when);
-extern REGISTRY_MACHINE *registry_machine_get(const char *machine_guid, time_t when);
-extern REGISTRY_MACHINE_URL *registry_machine_link_to_url(REGISTRY_MACHINE *m, REGISTRY_URL *u, time_t when);
+REGISTRY_MACHINE *registry_machine_find(const char *machine_guid);
+REGISTRY_MACHINE_URL *registry_machine_url_allocate(REGISTRY_MACHINE *m, REGISTRY_URL *u, time_t when);
+REGISTRY_MACHINE *registry_machine_allocate(const char *machine_guid, time_t when);
+REGISTRY_MACHINE *registry_machine_get(const char *machine_guid, time_t when);
+REGISTRY_MACHINE_URL *registry_machine_link_to_url(REGISTRY_MACHINE *m, REGISTRY_URL *u, time_t when);
 
 #endif //NETDATA_REGISTRY_MACHINE_H