stdin is already defined by stdio.h

Fix compiler warnings:

pn53x-tamashell.c: In function ‘main’:
pn53x-tamashell.c:79: warning: nested extern declaration of ‘__stdinp’
pn53x-tamashell.c:79: warning: redundant redeclaration of ‘__stdinp’
/usr/include/stdio.h:164: warning: previous declaration of ‘__stdinp’
was here

examples/pn53x-tamashell.c

@@ -76,7 +76,6 @@ int main(int argc, const char* argv[])
   uint8_t abtTx[MAX_FRAME_LEN];
   size_t szRx = sizeof(abtRx);
   size_t szTx;
-  extern FILE* stdin;
   FILE* input = NULL;
 
   if (argc >= 2) {