libsparse: Fix verbose_error string usage

Switch vnsprintf to StringPrintf to avoid off-by-one, and switch
sparse_read.c to cpp.

Test: fastboot flash normal image with smaller sparse limit

Change-Id: Ia399b167625deb271bfd0ee3273071306d71c4d4
diff --git a/libsparse/sparse_file.h b/libsparse/sparse_file.h
index 91a12e6..763f43f 100644
--- a/libsparse/sparse_file.h
+++ b/libsparse/sparse_file.h
@@ -17,6 +17,10 @@
 #ifndef _LIBSPARSE_SPARSE_FILE_H_
 #define _LIBSPARSE_SPARSE_FILE_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <sparse/sparse.h>
 
 struct sparse_file {
@@ -28,5 +32,8 @@
 	struct output_file *out;
 };
 
+#ifdef __cplusplus
+}
+#endif
 
 #endif /* _LIBSPARSE_SPARSE_FILE_H_ */