Only define the contents of a struct minitar inside minitar

This structure should be opaque for everyone using the library, and every public API just uses a pointer to it.

Makefile

@@ -6,7 +6,7 @@ SRC := src
 CC ?= gcc
 AR ?= ar
 CFLAGS ?= -O2 -Wall -Wextra
-CFLAGS := ${CFLAGS} -I.
+CFLAGS := ${CFLAGS} -I. -D_IN_MINITAR
 DESTDIR ?= /usr/local
 
 OBJS := $(OBJ)/tar.o \

minitar.h

@@ -3,10 +3,14 @@
 #include <stdio.h>
 #include <sys/types.h>
 
+#ifdef _IN_MINITAR
 struct minitar
 {
     FILE* stream;
 };
+#else
+struct minitar;
+#endif
 
 enum minitar_file_type
 {