LibC: Add mntent stub functions

These functions are required by Qt but since we don't really need them
for just basic apps, let's leave the implementation for later.

Userland/Libraries/LibC/mntent.cpp

@@ -16,4 +16,25 @@ struct mntent* getmntent(FILE*)
     TODO();
     return nullptr;
 }
+
+FILE* setmntent(char const*, char const*)
+{
+    dbgln("FIXME: Implement setmntent()");
+    TODO();
+    return nullptr;
+}
+
+int endmntent(FILE*)
+{
+    dbgln("FIXME: Implement endmntent()");
+    TODO();
+    return 0;
+}
+
+struct mntent* getmntent_r(FILE*, struct mntent*, char*, int)
+{
+    dbgln("FIXME: Implement getmntent_r()");
+    TODO();
+    return 0;
+}
 }

Userland/Libraries/LibC/mntent.h

@@ -24,5 +24,8 @@ struct mntent {
 };
 
 struct mntent* getmntent(FILE* stream);
+FILE* setmntent(char const* filename, char const* type);
+int endmntent(FILE* streamp);
+struct mntent* getmntent_r(FILE* streamp, struct mntent* mntbuf, char* buf, int buflen);
 
 __END_DECLS