MDEV-33750: Fix DPKG_GENSYMBOLS_CHECK_LEVEL so it actually takes effect

The way DPKG_GENSYMBOLS_CHECK_LEVEL was exported did not actually
have any effect on the build. Fix the syntax so that build will
indeed fail if there there are new symbols in new upstream version.

debian/rules

@@ -19,8 +19,10 @@ CFLAGS+=$(CPPFLAGS)
 CXXFLAGS+=$(CPPFLAGS)
 
 # Only do a strict symbol checking on Linux
+# https://manpages.debian.org/testing/dpkg-dev/dpkg-gensymbols.1.en.html
+# Level 4: Fails if some libraries have been introduced.
 ifneq (,$(filter linux,$(DEB_HOST_ARCH_OS)))
-    DPKG_GENSYMBOLS_CHECK_LEVEL := 4
+    export DPKG_GENSYMBOLS_CHECK_LEVEL = 4
 endif
 
 BUILDDIR := builddir