Rename library/ecp_alt.h to ecp_internal_alt.h

library/ecp_alt.h (declaring individual functions of the ECP module that can
be substituted, included when building the library with
MBEDTLS_ECP_INTERNAL_ALT enabled) clashes with ecp_alt.h (not provided,
declaring types of the ECP module when the whole implementation is
substituted, included when building the library with MBEDTLS_ECP_ALT enabled).
Depending on the search path during build, this can make MBEDTLS_ECP_ALT
unusable.

Rename library/ecp_alt.h to follow the naming convention of other alt headers:
MBEDTLS_XXX_ALT corresponds to xxx_alt.h.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index f450056..044bbe1 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -99,7 +99,7 @@
 #define mbedtls_free       free
 #endif
 
-#include "ecp_alt.h"
+#include "ecp_internal_alt.h"
 
 #if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
 #if defined(MBEDTLS_HMAC_DRBG_C)