Don't include <alloca.h>, it's no longer needed.

Relevant code was removed in 5d5e39f5d28f89d74a91cc2b1d6ef3b38c23fdc6.

Change-Id: I198844064030c04f88e5541f2bbaa29ae13d14bb
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://boringssl-review.googlesource.com/6521
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/bn/generic.c b/crypto/bn/generic.c
index 7fd4819..f6821bf 100644
--- a/crypto/bn/generic.c
+++ b/crypto/bn/generic.c
@@ -69,12 +69,6 @@
     (!defined(OPENSSL_X86_64) && !defined(OPENSSL_X86)) || \
     (defined(OPENSSL_X86_64) && defined(OPENSSL_WINDOWS))
 
-#if defined(OPENSSL_WINDOWS)
-#define alloca _alloca
-#else
-#include <alloca.h>
-#endif
-
 #ifdef BN_LLONG
 #define mul_add(r, a, w, c)             \
   {                                     \