vagrant/win32: reduce amount of threads

vagrant/win32: reduce amount of threads

Otherwise I got:
    win:     855 [main] sh 840 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: /bin/sh: /usr/bin/sed: Input/output error
    win: /bin/sh: line 1: /bin/sh: Input/output error
    win:   CC       bufferevent_pair.lo
    win: /bin/sh: fork: Input/output error
    win: make[1]: *** [Makefile:1722: buffer.lo] Error 126
    win: make[1]: *** Waiting for unfinished jobs....
    win:   CC       bufferevent_ratelim.lo
    win:     176 [main] sh 2440 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: /bin/sh: fork: Input/output error
    win:       0 [main] sh 3684 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: ./libtool: fork: Input/output error
    win: make[1]: *** [Makefile:1722: bufferevent_filter.lo] Error 254
    win:       0 [main] sh 1356 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: ./libtool: fork: Input/output error
    win:       0 [main] sh 3640 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: ./libtool: line 922: /usr/bin/sed: Input/output error
    win: ./libtool: fork: Input/output error
    win:       0 [main] sh 3444 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win:       0 [main] sh 3796 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: ./libtool: fork: Input/output error
    win:     693 [main] sh 304 fork: child -1 - CreateProcessW failed for 'C:\tools\cygwin\bin\sh.exe', errno 5
    win: ./libtool: fork: Input/output error
    win: ./libtool: fork: Input/output error
    win: ./libtool: line 1377: /usr/bin/sed: Input/output error
    win: :   error: Failed to create '.libs'