Make C API compatible with -Wstrict-prototypes

Change-Id: I265605e769a12a319d672c491c9ba2d89c02de7e
Reviewed-on: https://skia-review.googlesource.com/6477
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

include/c/sk_data.h

@@ -19,7 +19,7 @@ SK_C_PLUS_PLUS_BEGIN_GUARD
     Returns a new empty sk_data_t.  This call must be balanced with a call to
     sk_data_unref().
 */
-SK_API sk_data_t* sk_data_new_empty();
+SK_API sk_data_t* sk_data_new_empty(void);
 /**
     Returns a new sk_data_t by copying the specified source data.
     This call must be balanced with a call to sk_data_unref().

include/c/sk_paint.h

@@ -28,7 +28,7 @@ SK_C_PLUS_PLUS_BEGIN_GUARD
         maskfilter : NULL
         xfermode_mode : SRCOVER_SK_XFERMODE_MODE
 */
-SK_API sk_paint_t* sk_paint_new();
+SK_API sk_paint_t* sk_paint_new(void);
 /**
     Release the memory storing the sk_paint_t and unref() all
     associated objects.

include/c/sk_path.h

@@ -21,7 +21,7 @@ typedef enum {
 } sk_path_direction_t;
 
 /** Create a new, empty path. */
-SK_API sk_path_t* sk_path_new();
+SK_API sk_path_t* sk_path_new(void);
 /** Release the memory used by a sk_path_t. */
 SK_API void sk_path_delete(sk_path_t*);
 

include/c/sk_picture.h

@@ -19,7 +19,7 @@ SK_C_PLUS_PLUS_BEGIN_GUARD
     Create a new sk_picture_recorder_t.  Its resources should be
     released with a call to sk_picture_recorder_delete().
 */
-sk_picture_recorder_t* sk_picture_recorder_new();
+sk_picture_recorder_t* sk_picture_recorder_new(void);
 /**
     Release the memory and other resources used by this
     sk_picture_recorder_t.

include/c/sk_types.h

@@ -81,7 +81,7 @@ typedef enum {
 /**
     Return the default sk_colortype_t; this is operating-system dependent.
 */
-SK_API sk_colortype_t sk_colortype_get_default_8888();
+SK_API sk_colortype_t sk_colortype_get_default_8888(void);
 
 typedef struct {
     int32_t         width;