healthcheck: exclude unsupported syscalls from JS builds

Because the health check uses OS specific syscalls for determining the
available space on a disk that aren't available in JS/WASM builds, we
need to make sure we don't reference that code at all. Otherwise we
can't use parts of lnd as a library in projects that are being compiled
down to a WASM binary.

healthcheck/diskcheck.go

@@ -1,4 +1,4 @@
-// +build !windows,!solaris,!netbsd,!openbsd
+// +build !windows,!solaris,!netbsd,!openbsd,!js
 
 package healthcheck
 

healthcheck/diskcheck_js.go

@@ -0,0 +1,13 @@
+package healthcheck
+
+// AvailableDiskSpaceRatio returns ratio of available disk space to total
+// capacity.
+func AvailableDiskSpaceRatio(path string) (float64, error) {
+	return 0, fmt.Errorf("disk space check not supported in WebAssembly")
+}
+
+// AvailableDiskSpace returns the available disk space in bytes of the given
+// file system.
+func AvailableDiskSpace(path string) (uint64, error) {
+	return 0, fmt.Errorf("disk space check not supported in WebAssembly")
+}