pbp: pstitleimg sections are allowed also for single disc games

Multidisc games are stored inside the EBOOT.PBP file as
A)
pstitleimg
psisoimg
psisoimg
...

Single disk games can be stored in two different formats, with or
without a pstitleimg section, i.e. as

B)
pstitleimg
psisoimg

or

C)
psisoimg

Fix a conditional in duckstation that incorrectly flags B) as an error.
These types of EBOOT.PBP files can be created by the pop-fe utility.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

src/common/cd_image_pbp.cpp

@@ -423,7 +423,7 @@ bool CDImagePBP::Open(const char* filename, Common::Error* error)
         break;
     }
 
-    if (m_disc_offsets.size() < 2)
+    if (m_disc_offsets.size() < 1)
     {
       Log_ErrorPrintf("Invalid number of discs (%u) in multi-disc PBP file", static_cast<u32>(m_disc_offsets.size()));
       return false;