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, 9 insertions, 9 deletions

diff --git a/database/engine/rrdengine.h b/database/engine/rrdengine.h
index 4b383b6226..9a2e005831 100644
--- a/database/engine/rrdengine.h
+++ b/database/engine/rrdengine.h
@@ -256,14 +256,14 @@ struct rrdengine_instance {
     struct rrdengine_statistics stats;
 };
 
-extern void *dbengine_page_alloc(void);
-extern void dbengine_page_free(void *page);
-
-extern int init_rrd_files(struct rrdengine_instance *ctx);
-extern void finalize_rrd_files(struct rrdengine_instance *ctx);
-extern void rrdeng_test_quota(struct rrdengine_worker_config* wc);
-extern void rrdeng_worker(void* arg);
-extern void rrdeng_enq_cmd(struct rrdengine_worker_config* wc, struct rrdeng_cmd *cmd);
-extern struct rrdeng_cmd rrdeng_deq_cmd(struct rrdengine_worker_config* wc);
+void *dbengine_page_alloc(void);
+void dbengine_page_free(void *page);
+
+int init_rrd_files(struct rrdengine_instance *ctx);
+void finalize_rrd_files(struct rrdengine_instance *ctx);
+void rrdeng_test_quota(struct rrdengine_worker_config* wc);
+void rrdeng_worker(void* arg);
+void rrdeng_enq_cmd(struct rrdengine_worker_config* wc, struct rrdeng_cmd *cmd);
+struct rrdeng_cmd rrdeng_deq_cmd(struct rrdengine_worker_config* wc);
 
 #endif /* NETDATA_RRDENGINE_H */