[libtremor] added alloca.h '#include's

Was getting 'undefined reference' errors in the upcoming SDL2 build
(GCC 5.4.0 on Lubuntu 16.04)

core/tremor/codebook.c

@@ -15,6 +15,7 @@
 
  ********************************************************************/
 
+#include <alloca.h>
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>

core/tremor/floor0.c

@@ -15,6 +15,7 @@
 
  ********************************************************************/
 
+#include <alloca.h>
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>

core/tremor/info.c

@@ -18,6 +18,7 @@
 /* general handling of the header and the vorbis_info structure (and
    substructures) */
 
+#include <alloca.h>
 #include <stdlib.h>
 #include <string.h>
 #include <ctype.h>

core/tremor/mapping0.c

@@ -15,6 +15,7 @@
 
  ********************************************************************/
 
+#include <alloca.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

core/tremor/res012.c

@@ -15,6 +15,7 @@
 
  ********************************************************************/
 
+#include <alloca.h>
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>

core/tremor/sharedbook.c

@@ -15,6 +15,7 @@
 
  ********************************************************************/
 
+#include <alloca.h>
 #include <stdlib.h>
 #include <math.h>
 #include <string.h>