dropwatch: add missing dependencies

dropwatch requires thread support because it selects libnl that
depends on thread support. Also dropwatch cannot build on AArch64,
because it selects the target binutils package which isn't (yet)
available on AArch64.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/dropwatch/Config.in

@@ -1,10 +1,15 @@
 config BR2_PACKAGE_DROPWATCH
 	bool "dropwatch"
 	select BR2_PACKAGE_BINUTILS
+	depends on !BR2_aarch64 # binutils
 	select BR2_PACKAGE_READLINE
 	select BR2_PACKAGE_LIBNL
+	depends on BR2_TOOLCHAIN_HAS_THREADS # libnl
 	help
 	  Dropwatch is an interactive utility for monitoring and
 	  recording packets that are dropped by the kernel
 
 	  https://fedorahosted.org/dropwatch/
+
+comment "dropwatch requires threading support in toolchain"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS