Fix double space in include argument

The double space is confusing compdb.go.  compdb.go should be fixed,
but the double space is unnecessary so remove it.

Also make -isystem consistently followed by a space.

Bug: 117124308
Test: m checkbuild
Change-Id: I5ce7530d2ef66be8d8285e252d60a39299984a06
diff --git a/cc/ndk_prebuilt.go b/cc/ndk_prebuilt.go
index 258d6bd..2a7e657 100644
--- a/cc/ndk_prebuilt.go
+++ b/cc/ndk_prebuilt.go
@@ -137,7 +137,7 @@
 		ctx.ModuleErrorf("NDK prebuilt libraries must have an ndk_lib prefixed name")
 	}
 
-	ndk.exportIncludes(ctx, "-isystem")
+	ndk.exportIncludes(ctx, "-isystem ")
 
 	libName := strings.TrimPrefix(ctx.ModuleName(), "ndk_")
 	libExt := flags.Toolchain.ShlibSuffix()