package/dropbear: fix typo

There was a missing space between the append-assignment operator, and
the appended list of licenses.

Even though inconsequential technically speaking, we always use spaces
around operators elsewhere in the code.

So be it here too.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/dropbear/dropbear.mk

@@ -79,7 +79,7 @@ DROPBEAR_POST_EXTRACT_HOOKS += DROPBEAR_ENABLE_REVERSE_DNS
 endif
 
 ifeq ($(BR2_PACKAGE_DROPBEAR_SMALL),y)
-DROPBEAR_LICENSE +=, Unlicense, WTFPL
+DROPBEAR_LICENSE += , Unlicense, WTFPL
 DROPBEAR_LICENSE_FILES += libtommath/LICENSE libtomcrypt/LICENSE
 DROPBEAR_CONF_OPTS += --disable-zlib --enable-bundled-libtom
 else