Use mkstemp instead of tmpnam in sstregistertool.cc

[berquist]

As part of #983 I unwittingly used a deprecated function:

/Users/ejberqu/development/sst/github/sst-core/src/sst/core/sstregistertool.cc:153:39: warning: 'tmpnam' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of tmpnam(3), it is highly recommended that you use mkstemp(3) instead. [-Wdeprecated-declarations]
    const auto        tempfile = std::tmpnam(nullptr);
                                      ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/stdio.h:189:1: note: 'tmpnam' has been explicitly marked deprecated here
__deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of tmpnam(3), it is highly recommended that you use mkstemp(3) instead.")
^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:215:48: note: expanded from macro '__deprecated_msg'
        #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))

[berquist]

Done in #993