Amalgamation is forced to include missing sqlite-vec-diskann.c.

The define is always set: ``` #ifndef SQLITE_VEC_ENABLE_DISKANN #define SQLITE_VEC_ENABLE_DISKANN 1 #endif ``` So this fails in the amalgamation: ``` #if SQLITE_VEC_ENABLE_DISKANN #include "sqlite-vec-diskann.c" #else static int vec0_all_columns_diskann(vec0_vtab *p) { (void)p; return 0; } #endif ```