Include <string.h> for various string functions.  This avoids implicit
function declarations and resulting build failures with future
compilers.

Submitted upstream: <https://github.com/rpm-software-management/drpm/pull/20>

diff --git a/src/drpm_write.c b/src/drpm_write.c
index 8c16a17cb0e01898..87da9724a84ee72a 100644
--- a/src/drpm_write.c
+++ b/src/drpm_write.c
@@ -23,6 +23,7 @@
 
 #include <stdio.h>
 #include <stdint.h>
+#include <string.h>
 #include <fcntl.h>
 #include <openssl/md5.h>
 #include <rpm/rpmlib.h>