linux/err.h: add PTR_ERR_OR_ZERO

Add missing PTR_ERR_OR_ZERO implementation. Also add a bunch of
generated files to .gitignore.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
diff --git a/include/linux/err.h b/include/linux/err.h
index a3db4cf..1b1dafb 100644
--- a/include/linux/err.h
+++ b/include/linux/err.h
@@ -30,4 +30,9 @@
 	return (!ptr) || IS_ERR_VALUE((unsigned long)ptr);
 }
 
+static inline long PTR_ERR_OR_ZERO(const void *ptr)
+{
+	return IS_ERR(ptr) ? PTR_ERR(ptr) : 0;
+}
+
 #endif
diff --git a/src/.gitignore b/src/.gitignore
index e0292b1..fddb29b 100644
--- a/src/.gitignore
+++ b/src/.gitignore
@@ -1,2 +1,4 @@
 *.o
 *.a
+/libbpf.pc
+/libbpf.so*