Fix build with -fno-common

Part of Patch from https://src.fedoraproject.org/rpms/xfce4-cpufreq-plugin/raw/master/f/xfce4-cpufreq-plugin-1.2.1-gcc10-common.patch

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/panel-plugin/xfce4-cpufreq-plugin.c
+++ b/panel-plugin/xfce4-cpufreq-plugin.c
@@ -37,7 +37,8 @@
 #include "xfce4-cpufreq-linux.h"
 #endif /* __linux__ */
 
-
+CpuFreqPlugin *cpuFreq;
+GtkCssProvider *currentProvider;
 
 void
 cpufreq_label_set_font (void)
--- a/panel-plugin/xfce4-cpufreq-plugin.h
+++ b/panel-plugin/xfce4-cpufreq-plugin.h
@@ -93,9 +93,9 @@ typedef struct
   gint timeoutHandle;
 } CpuFreqPlugin;
 
-CpuFreqPlugin *cpuFreq;
+extern CpuFreqPlugin *cpuFreq;
 
-GtkCssProvider *currentProvider;
+extern GtkCssProvider *currentProvider;
 
 G_BEGIN_DECLS