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, 8 insertions, 8 deletions

diff --git a/libnetdata/worker_utilization/worker_utilization.h b/libnetdata/worker_utilization/worker_utilization.h
index b64d76a9ed..aed893ed2d 100644
--- a/libnetdata/worker_utilization/worker_utilization.h
+++ b/libnetdata/worker_utilization/worker_utilization.h
@@ -14,18 +14,18 @@ typedef enum {
     WORKER_METRIC_INCREMENTAL = 3,
 } WORKER_METRIC_TYPE;
 
-extern void worker_register(const char *workname);
-extern void worker_register_job_name(size_t job_id, const char *name);
-extern void worker_register_job_custom_metric(size_t job_id, const char *name, const char *units, WORKER_METRIC_TYPE type);
-extern void worker_unregister(void);
+void worker_register(const char *workname);
+void worker_register_job_name(size_t job_id, const char *name);
+void worker_register_job_custom_metric(size_t job_id, const char *name, const char *units, WORKER_METRIC_TYPE type);
+void worker_unregister(void);
 
-extern void worker_is_idle(void);
-extern void worker_is_busy(size_t job_id);
-extern void worker_set_metric(size_t job_id, NETDATA_DOUBLE value);
+void worker_is_idle(void);
+void worker_is_busy(size_t job_id);
+void worker_set_metric(size_t job_id, NETDATA_DOUBLE value);
 
 // statistics interface
 
-extern void workers_foreach(const char *workname, void (*callback)(
+void workers_foreach(const char *workname, void (*callback)(
                                                       void *data
                                                       , pid_t pid
                                                       , const char *thread_tag