sign_rpm.bbclass: fix variable assignment whitespace

Fixes:
WARNING: meta/classes/sign_rpm.bbclass:36 has a lack of whitespace
         around the assignment: 'RPM_SIGN_PACKAGES='1''

(From OE-Core rev: d393539e03b60bf299e2d1c8ac781e0c6f7787b1)

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/sign_rpm.bbclass

@@ -33,7 +33,7 @@
 
 inherit sanity
 
-RPM_SIGN_PACKAGES='1'
+RPM_SIGN_PACKAGES = '1'
 RPM_SIGN_FILES ?= '0'
 RPM_GPG_BACKEND ?= 'local'
 # SHA-256 is used by default