docs: add libssh2_userauth_publickey_frommemory.3 to dist

The function and man page were added in commit 18cfec8336e
diff --git a/docs/Makefile.am b/docs/Makefile.am
index c5d0b90..dc11f2f 100644
--- a/docs/Makefile.am
+++ b/docs/Makefile.am
@@ -169,4 +169,5 @@
 	libssh2_userauth_publickey.3 \
 	libssh2_userauth_publickey_fromfile.3 \
 	libssh2_userauth_publickey_fromfile_ex.3 \
+	libssh2_userauth_publickey_frommemory.3 \
 	libssh2_version.3