Workaround pthread issue with uClibc-0.9.29

From hp-utils-0.3.2/README:

  04  KNOWN PROBLEMS
  uClibc 0.9.29 has issues with POSIX threads and this breaks hp-utils.

Explicitly linking against pthread makes it work,
s. https://github.com/Freetz/freetz/commit/458846f1828ad41fe8027666a21ab59a9155eccb

--- Makefile.hp-utils
+++ Makefile.hp-utils
@@ -1,4 +1,4 @@
-LDLIBS += -lhp-utils
+LDLIBS += -lhp-utils -lpthread
 RELEASE = 0.3.2
 DEFS = -DEXTRA_VERSION="\"\nfrom hp-utils $(RELEASE)\""
 LIB = libhp-utils.so