package/pkg-cargo.mk: fix debug build

There is no --debug mode for cargo resulting in the following build
failure in ripgrep since its conversion to cargo infrastructure in
commit 342fd3e7350479dc368541ac77a5130ebb53194c:

error: Found argument '--debug' which wasn't expected, or isn't valid in this context

Fixes:
 - http://autobuild.buildroot.org/results/772ebdd6a39950457a59419bdc1376cb0e0a3611

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/pkg-cargo.mk

@@ -104,7 +104,7 @@ define $(2)_BUILD_CMDS
 		$$($(2)_CARGO_ENV) \
 		cargo build \
 			--offline \
-			$$(if $$(BR2_ENABLE_DEBUG),--debug,--release) \
+			$$(if $$(BR2_ENABLE_DEBUG),,--release) \
 			--manifest-path Cargo.toml \
 			--locked \
 			$$($(2)_CARGO_BUILD_OPTS)