s_symbol: Close memory leak of symbol name.

s_symbol: Close memory leak of symbol name.

Easily done now that s_expression is allocated with talloc. Simply
switch from new to talloc_strdup and the job is done.

This closes the great majority (11263) of the remaining leaks in the
glsl-orangebook-ch06-bump.frag test:

	total heap usage: 55,623 allocs, 55,546 frees
	(was 44,283 frees)

This test is now 99.86% leak-free.