media: venus: place extern venus_fw_debug on a header file

Sparse warns about this symbol:

drivers/media/platform/qcom/venus/hfi_venus.c:133:5: warning: symbol 'venus_fw_debug' was not declared. Should it be static?

Because hfi_venus.c doesn't include a header file with the
extern. So, move it to core.h, with is included by both
hfi_venus.c and dbgfs.c.

This way, if something changes with it, warnings or errors
will be produced.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/platform/qcom/venus/core.h

···
 #define VIDC_VCODEC_CLKS_NUM_MAX	2
 #define VIDC_PMDOMAINS_NUM_MAX		3
 
+extern int venus_fw_debug;
+
 struct freq_tbl {
 	unsigned int load;
 	unsigned long freq;

drivers/media/platform/qcom/venus/dbgfs.c

···
 
 #include "core.h"
 
-extern int venus_fw_debug;
-
 void venus_dbgfs_init(struct venus_core *core)
 {
 	core->root = debugfs_create_dir("venus", NULL);