rules.mk: fix syntax error

Fix broken assignment operator added in a previous commit.

Fixes db73ec9f51 ("rules.mk: add INSTALL_SUID macro")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

rules.mk

@@ -281,7 +281,7 @@ PATCH:=patch
 PYTHON:=python
 
 INSTALL_BIN:=install -m0755
-INSTALL_SUID:install -m4755
+INSTALL_SUID:=install -m4755
 INSTALL_DIR:=install -d -m0755
 INSTALL_DATA:=install -m0644
 INSTALL_CONF:=install -m0600