[osg-submissions] Fix warnings for TXP plugin

[Mario Valle]

Fixed "warning: deprecated conversion from string constant to 'char*'"
for the TXP plugin.
Tested on Linux Suse 10.4 32bit with g++ 4.2.1

-- 
Ing. Mario Valle
Data Analysis and Visualization Services         | http://www.cscs.ch/~mvalle
Swiss National Supercomputing Centre (CSCS)      | Tel:  +41 (91) 610.82.60
v. Cantonale Galleria 2, 6928 Manno, Switzerland | Fax:  +41 (91) 610.82.82
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