Add boost chrono lib for windows builds (required for boost 1.50)

src/makefile.linux-mingw

@@ -25,6 +25,7 @@ LIBS= \
  -l boost_filesystem-mt-s \
  -l boost_program_options-mt-s \
  -l boost_thread_win32-mt-s \
+ -l boost_chrono-mt-s \
  -l db_cxx \
  -l ssl \
  -l crypto

src/makefile.mingw

@@ -20,6 +20,7 @@ LIBS= \
  -l boost_filesystem-mgw45-mt-s-1_50 \
  -l boost_program_options-mgw45-mt-s-1_50 \
  -l boost_thread-mgw45-mt-s-1_50 \
+ -l boost_chrono-mgw45-mt-s-1_50 \
  -l db_cxx \
  -l ssl \
  -l crypto