lib: Hide struct fp_minutiae from the docs

It's only used internally.

doc/libfprint-sections.txt

@@ -235,7 +235,6 @@ fp_img_driver
 <SECTION>
 <INCLUDE>fpi-img.h</INCLUDE>
 <FILE>fpi-img</FILE>
-fp_minutiae
 FpiImgFlags
 
 fpi_img_new

libfprint/fpi-img.h

@@ -67,7 +67,9 @@ struct fp_img {
 	int height;
 	size_t length;
 	FpiImgFlags flags;
+	/*< private >*/
 	struct fp_minutiae *minutiae;
+	/*< public >*/
 	unsigned char *binarized;
 	unsigned char data[0];
 };