Fixes XML undefined symbol error

libudunitst2 has an undefined symbol in the symbol table:
```
(blank)[lcampbell@linuxhost lib]$ nm libudunits2.so.0.1.0
                 U XML_ErrorString
                 U XML_GetBase
                 U XML_GetCurrentColumnNumber
                 U XML_GetCurrentLineNumber
                 U XML_GetErrorCode
                 U XML_Parse
                 U XML_ParserCreate
                 U XML_ParserFree
                 U XML_SetBase
                 U XML_SetCharacterDataHandler
                 U XML_SetElementHandler
                 U XML_SetXmlDeclHandler
                 U XML_StopParser
```
By adding libexpat to the library dependency it should ensure that these
symbols are referenced at runtime.