pt-br: fix voice's warnings "mbrola: Warning: r-... unkown, replaced with _-_"

E.g. for 'port' this goes through following transformations:

1. written 'port' to phonetic [p'o*@-tS] for eSpeak NG
2. espeak [*] was mapped to MBROLA [r] and [@-] was just skipped
resulting [rt] which is not allowed diphone in br1 voice.

Description of fix:
mapping phonetic [*@-] to [r2] (and skipping [@-]) makes [r2-...] which is
in diphone database. That fixed warnings:

mbrola: Warning: r-t unkown, replaced with _-_
mbrola: Warning: r-d unkown, replaced with _-_
mbrola: Warning: r-n unkown, replaced with _-_
mbrola: Warning: r-m unkown, replaced with _-_
mbrola: Warning: r-s unkown, replaced with _-_

phsource/mbrola/ptbr

@@ -11,6 +11,7 @@ volume 15
 0   ;   NULL   0   NULL
 0   g-  NULL   0   NULL
 0   @-  NULL   0   NULL
+0   *   @-     0   r2
 8   *   j      0   r
 0   *   VWL    0   r
 0   *   NULL   0   r2