glsl_to_tgsi: copy reladdr in st_src_reg(st_dst_reg) constructor

This is a glsl_to_tgsi port of commit f7cd9a858c04.
diff --git a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
index f66e240..ba4074e 100644
--- a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
+++ b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
@@ -174,7 +174,7 @@
    this->index = reg.index;
    this->swizzle = SWIZZLE_XYZW;
    this->negate = 0;
-   this->reladdr = NULL;
+   this->reladdr = reg.reladdr;
 }
 
 st_dst_reg::st_dst_reg(st_src_reg reg)